Sikhism is the fifth-largest world religion. It was founded by Guru Nanak Dev in 16th-century India. The “guru-hood” or “prophet-hood” was passed down to 9 other gurus. The 10th guru finalised the Sikh holy scripture (Sri Guru Granth Sahib) and declared it the last or final (11th) guru. The Sikh equivalent of the Vatican, the Akaal Takhat, dictates the interpretation of the scripture and the way Sikh practises are to be performed. The Sikh Rehat Maryada (code of conduct) defines what a Sikh is and how a Sikh is to follow their religion.
A Sikh is defined, according to the Sikh Rehat Maryada as one who fulfills the following criteria:
- Believes in one immortal deity
- Follows the teachings of the ten gurus
- Accepts the Guru Granth Sahib (the Sikh holy book) as the final guru
- Has been baptised
- Does not owe allegiance to other religions
Most self-professed Sikhs do not fulfil these criteria i.e. though they are monotheist, they do not follow the teachings of the gurus and/or are not baptised. Therefore most self-professed Sikhs are "cultural Sikhs" or Sehajdhari Sikhs (non-observant Sikhs).
The Sikh religion promotes belief in a deity that is more diluted in its interventionism and anthropomorphic characteristic compared to the deity in Christianity and Islam (though Sikhs believe they worship the same deity as the Abrahamic one). The god concept in the Sikh religion is not easily discernible due to the deity and hovers between deism and pantheism. In the Guru Granth Sahib, the deity is referred to as “Waheguru" (meaning wonderful teacher) and other names such as Allah and Raam are also used.
Sikhs congregate in a "Gurdwara" (house of the guru, a Sikh version of a church or mosque) mostly on Sundays though it is said Gurdwara attendance must be daily.  Non-Sikhs are allowed to enter the Gurdwara as long as the rules are followed (removing shoes, covering the head, not being intoxicated, etc). A Gurdwara mainly consists of a main room where religious teachings take place, and a dinner hall ("langar hall") in which free vegetarian food is distributed without discrimination (i.e. people of different religions or the irreligious). 
Key Sikh literature
In "Sikhi", the Sri Guru Granth Sahib (SGGS for short) and the Rehat Maryada are the key sources of information regarding the religion. The SGGS predominantly contains love/devotional poetry about god, the Maryada predominantly contains the practises/rituals that Sikhs are to engage in.
Sri Guru Granth Sahib (SGGS for short): The composition of poetry from various authors, finalized several hundred years ago by the tenth guru. Though some of the authors were from Hindu or Muslim backgrounds, they shared the same worldview as the gurus and they behaved in accordance with the Sikh religion. No mention is made of the tenth guru in the SGGS, so external literature or sources such as the Sikh Rehat Maryada are also of importance to the religion.
Sikh Rehat Maryada: Sikh code of conduct- this was authored almost a hundred years ago by a committee of Sikh scholars on what they felt was consistent with the teachings in the Guru Granth Sahib as well as the life styles the Sikh gurus and other relevant prophets. The Sikh Rehat Maryada is of equal importance as the SGGS to the Sikh religion. The Rehat Maryada contains the following key information about the Sikh religion:
- Birth (naming), baptism (or conversion), marriage and funeral ceremonies.
- How to read the SGGS and how to perform prayers
- Dos and Dont's of the religion
- The appropriate contruction or anatomy of gurdwaras (Sikh temples), how the temples and various religious programs are to be operated.
Sakhees: Life stories of the Sikh gurus. Authored by various people, not always clear (sometimes by "witnesses", companions or followers).
Sikh way of life
As mentioned above, a Sikh is one who lives in accordance to the principles/teachings of the 10 gurus and emulating the three pillars of Sikhism:
Sikhs are expected to be baptised and lead a life in which one wakes at 5am, bathes, prays, and adheres to the teaching in the Sri Guru Granth Sahib.
Deadly sins (bujjar kurehat)
According to the Sikh Rehat Maryada, there are five sins that a Sikh must not commit, committing any of these acts makes one an apostate from the Sikh religion:
- Hair removal. Cutting, shaving, trimming and waxing are not permitted- facial, armpit and pubic hair are meant to remain intact.
- Eating ritually slaughtered meat (i.e. halal, kosher). There is a great deal of controversy regarding meat eating in Sikhism, many Sikhs view eating meat of any sort as a deadly sin.
- Engaging in extramarital sex.
- Consuming drugs or alcohol.
Sikhs who have committed the above offenses are required to confess to the Sikh community, who then decide the punishment- community service is common.
The five Ks (panj kakkar)
These are the five "must-have" clothing items for Sikhs- they must always have these on, even in death. Most of the five Ks were conceived during the later development of the Sikh religion, in response to Mughal (Muslim) aggression, in which the Sikh religion began advocating the carrying of arms/weapons. The idea behind the 5Ks is for Sikhs to maintain perpetual readiness for battle.
- Untrimmed/unaltered hair (kés)- long hair is seen as submission/acceptance of the deity's design.
- Comb (kunga)- to maintain the hair.
- Bangle (kara)- this items is meant to serve two functions: symbolically as an article of faith to remind the follower of their duties and also the omnipresence of the deity (symbolised by the circle), as a weapon and as protection from attack. Bangles are present in various designs and can be blunt or razor sharp. The bangle can provide a degree of protection for the wrist and some designs permit it to be used as a sharp “frizbee”. The bangle is to be worn on the wrist (sometimes following gender guidelines or on the dominant arm) and must be made of iron or steel and not gold.
- Kirpan (sword)- which may be symbolic miniature or a full sized weapon. Originally, the kirpan is not meant to be just a “symbol” or article of the Sikh religion as its key purpose is as a weapon for defensive or offensive purposes. The kirpan may also be used for other functions such as “blessing food” during ceremonies.
- Baggy (or boxer) shorts (kashera)- the kashera must always be worn even while bathing or having intercourse- "constant preparedness" (for battle) being used as a justification and are comparable in function (though not design) to Mormon underwear.
Though the Sikh religion staunchly opposes the caste system, a more diluted form of the caste system has been implemented by the followers. This diluted "caste system" informs which temples the follower can go to, what practices to adhere to as well as selecting marriage partners. It should be noted that a there is a distinction between a caste and a sect in Sikhi- a person is born into a caste (hereditary) whereas a sect one can join and leave at will regardless of lineage.
Castes: Ahluwalia, Arora, Bhatia, Bhatra, Jatt, Kamboj, Khatri, Kshatriya, Lubana, Majabi, Rajpoot/Rajput, Ramdasia, Ramgharia, Ravidasi, Saini, Suniara, Tonk Khatri,
Sects: 3HO, Akhand Kirtani Jatha, Namdhari, Nihang, Nirankari, Radha Swami,
Beliefs and practices
Afterlife and salvation
Sikhism puts forward the idea that the human form is a chance to escape perpetual reincarnation by living according to the ways of Sikhism, non-Sikhs continue to undergo reincarnation. Sikhs believe that the suffering one experiences is due to the sins committed in a previous life. Though it is expected for Sikhs to maintain a constant lifestyle adhering to a set of principles, a form of salvation similar to Christianity is present within the scripture to escape reincarnation.
"ਬਿਨੁ ਸਤਗੁਰ ਸੁਖੁ ਨ ਪਾਵਈ ਫਿਰਿ ਫਿਰਿ ਜੋਨੀ ਪਾਹਿ ॥੩॥"
"But without the True Guru, you will not find peace; you will be reincarnated over and over again." SGGS p26
"ਜਿਨ ਕਉ ਪ੍ਰੀਤਿ ਨਾਹੀ ਹਰਿ ਸੇਤੀ ਤੇ ਸਾਕਤ ਮੂੜ ਨਰ ਕਾਚੇ ॥ ਤਿਨ ਕਉ ਜਨਮੁ ਮਰਣੁ ਅਤਿ ਭਾਰੀ ਵਿਚਿ ਵਿਸਟਾ ਮਰਿ ਮਰਿ ਪਾਚੇ ॥੨॥"
"Those who are not in love with the Lord are foolish and false - they are faithless cynics. They suffer the most extreme agonies of birth and death (reincarnation); they die over and over again, and they rot away in manure (dirt)." SGGS p169
"ਕਾਮਿ ਕਰੋਧਿ ਮੋਹਿ ਵਸਿ ਕੀਆ ਕਿਰਪਨ ਲੋਭਿ ਪਿਆਰੁ ॥ ਚਾਰੇ ਕਿਲਵਿਖ ਉਨਿ ਅਘ ਕੀਏ ਹੋਆ ਅਸੁਰ ਸੰਘਾਰੁ ॥ ਪੋਥੀ ਗੀਤ ਕਵਿਤ ਕਿਛੁ ਕਦੇ ਨ ਕਰਨਿ ਧਰਿਆ ॥ ਚਿਤਿ ਆਵੈ ਓਸੁ ਪਾਰਬ੍ਰਹਮੁ ਤਾ ਨਿਮਖ ਸਿਮਰਤ ਤਰਿਆ ॥੪॥"
"When you are under the power of sexual desire, anger and worldy attachment, or a greedy miser in love with your wealth; if you have committed the four great sins and other mistakes; even if you are a murderous fiend who has never taken the time to listen to sacred books, hymns and poetry- if you then come to remember the Supreme Lord God, and contemplate Him, even for a moment, you shall be saved." SGGS p70
"ਰਸਨਾ ਉਚਰੈ ਗੁਣਵਤੀ ਕੋਇ ਨ ਪੁਜੈ ਦਾਨੁ ॥"
"The tongue which chants these Praises is worthy; there is no charity equal to this." SGGS p49
"ਪੁੰਨ ਦਾਨ ਚੰਗਿਆਈਆ ਬਿਨੁ ਸਾਚੇ ਕਿਆ ਤਾਸੁ ॥"
"You may give donations to charity, and perform good deeds, but without the True One, what is the use of it all?" SGGS p56
"ਕਿਛੁ ਪੁੰਨ ਦਾਨ ਅਨੇਕ ਕਰਣੀ ਨਾਮ ਤੁਲਿ ਨ ਸਮਸਰੇ ॥"
"Giving donations to charity, and performing various religious rituals are not equal to the contemplation of the Naam." SGGS p566
The religion condemns palm reading, astrology and magic in general- but does not go to the point of advocating killing of those who engage in activities. Instead, the Sikh gurus appeared to have had a "rationalist streak", by exposing those who used astrology and magic to extract money from the gullible. However, this general condemnation for superstition does not extend to reincarnation, ghosts and angels which are taught to exist according to Sikh scripture.
Attitude towards science
Sikhs in general do not view evolution, abiogenesis or the big bang as contradictory to the Sikh religion. The Sikh scripture has no mention of Adam and Eve, Noah or the flood. It is possible for Sikhs to have objections to science, archaeology, etc, if it challenges any of the Sikh doctrines or stories, etc. In such cases the religious belief takes precedence.
Though critical of some aspects of many religions (i.e. the caste system in Hinduism, burial vs. cremation in Islam), the Sikh religion does not bear enmity against people of other faiths. Citations from the SGGS are often used to state that Hindus should be good Hindus, and Muslims should be good Muslims- how this would be feasible considering the opposed views that these religions have with respect to each other is not something well explained in Sikhism. Often Sikhs use the 9th guru (Tegh Bahadhur) as an example of a Sikh standing up for the rights of non-Sikhs to being entitled to their own religious views (or freedom of religion).
According to the Sikh Rehat Maryada, interfaith marriages between Sikhs and non-Sikhs are not permitted and special emphasis is placed on Sikh women marrying non-Sikh males (which is related to child-bearing capabilities to propogate the religion). Despite this, Sikhs marrying non-Sikhs do occur but are rare. To challenge this, devout Sikhs have attempted to sabotage marriage ceremonies performed in gurdwaras to "preserve" the Sikh tradition when an interfaith marriage is about to occur.
Apostasy, shunning and atheism
There is no capital punishment for leaving the Sikh religion; no violence is advocated in the Sikh literature against those who leave the faith. Similarly, those who left the Sikh army even during warfare were not punished. Apostates (in Punjabi the word is "pateet") are seen as "lost" or "fallen" from the righteous path. However, those who leave the religion are meant to be shunned/ostracised and expelled from the family house and community. They are to be avoided and all contact must be broken from them (according to the Sikh Rehat Maryada). Committing any one of the five deadly sins in the Sikh religion renders one automatically an apostate.
Sikhs must not have any associations with those who would marry off their sons or daughters in matrimony for money, one who participates in "honour" killings, infanticide. It should also be noted that this list extends to:
- Sikhs that have eaten from a non-Sikhs plate
- One who changes their hair color
- Those who have used drugs (i.e. alcohol, marijuana, cocaine)
- Anyone who generally engages in practices which contradict the teaching of the gurus
Though the Sikh scriptures are not inherently violent, a few questionable verses exist which need further examination regarding atheists:
ਜੋ ਸਿਰੁ ਸਾਂਈ ਨਾ ਨਿਵੈ ਸੋ ਸਿਰੁ ਦੀਜੈ ਡਾਰਿ ॥
"Chop off that head which does not bow to the Lord." SGGS p89
ਜੋ ਸਿਰੁ ਸਾਂਈ ਨਾ ਨਿਵੈ ਸੋ ਸਿਰੁ ਕਪਿ ਉਤਾਰਿ ॥
ਜੋ ਸਿਰੁ ਸਾਈ ਨਾ ਨਿਵੈ ਸੋ ਸਿਰੁ ਕੀਜੈ ਕਾਂਇ ॥
ਕੁੰਨੇ ਹੇਠਿ ਜਲਾਈਐ ਬਾਲਣ ਸੰਦੈ ਥਾਇ ॥
"The head which does not bow to the Lord - chop off and remove that head. That head which does not bow to the Lord - what is to be done with that head? Put it in the fireplace, instead of firewood." SGGS p1381
Though Sikhism is mild in its treatments towards apostates and apostasy, independent or critical thinking over religious dogma is not encouraged.
Depiction of Gurus
Protests were mounted by Sikhs against a biopic movie about Guru Nanak, the founder of Sikhism. They objected to any depiction of Guru Nanak.  Even depicting the guru's close family is controversial. 
- "This is really important as Sikh Gurus are not limited to just being humans. Such movies will just be opening the door to people to draw our Guru in different forms that could affect His status in the hearts of his people. "
The Langar Hall (dinner hall) fights: disagreements over having chairs and tables.
Meat consumption: some view Sikhism as vegetarian whereas others in the Sikh community only see abstinence from halal and kosher meat as justified
Akhand Kirtani Jatha: A Sikh sect that believes part of the SGGS is not authentic.
Golden Temple sword fight: fight at the holiest of Sikh shrines on the anniversary of its attack. 
Hitman 2: Supposed Sikh temples are present in the game.
Conflicts with other religions
Though no emnity or intolerance of other religions is preached in the scripture (rather, unity through the similar "spiritual" goals), it is not uncommon for there to be conflicts between Sikh vs Muslims, and Sikh vs Hindus mostly due to historical events (ie genocide or discrimination being perpetrated against Sikhs). This has resulted in many Sikhs parents raising children with a disposition towards viewing Muslims (and to a lesser extent Hindus) as suspicious and of little good will. The common teaching is that Hindus are trying to dilute Sikhs heritage and religion (there is some basis for this but this stems from Hindu nationalists and not Hindus in general), and that Muslims target girls or young women for conversion. Conflicts between Sikhs and other religions such as Christianity and Buddhism are less pronounced, possible due to being exposed to the followers of these religions in developed nations where the religious is more progressive and tolerant.
Proofs of god's existence
The Sikh scripture puts forward various arguments to there being a deity which include arguments from justice, meaning of life, and fine tuning, which are common. The moral argument which is frequently used by Christians and Muslims, is seldom used by Sikhs. This may be due to the Sikh religion teaching that one should not expect rewards for moral behaviour but should be good for goodness sake.
So-called scientific miracles which are typically used by Muslim apologists (and to a lesser extent, Christian apologists) are also used by Sikhs . A common example would be to claim that the big bang theory is mentioned in the Guru Granth Sahib, predating Hubble's discovery. This is done by using slight-of-hand tactics where the words expressing the vastness of the universe, are reinterpreted to vaguely reference the expansion of the universe (example).
ਕੀਤਾ ਪਸਾਉ ਏਕੋ ਕਵਾਉ ॥
"You created the vast expanse of the universe with one word." SGGS p3
ਬਿਨੁ ਹਰਿ ਗੁਰ ਪ੍ਰੀਤਮ ਜਨਮੁ ਬਾਦਿ ॥
"Without the Beloved Lord and Master, life is meaningless." SGGS p1190
Sikhs are generally characterised by their turbans (which cover the long hair) and beards. To many Sikhs, hair is seen as more important than any limb on the body. Many would prefer death than to have their hair altered in any way, and there have been notable Sikh martyrs who at the threat of death or pain preferred death to seeing their hair cut. Cutting of hair is seen solely due to fashion and any notions of altering or trimming hair for practical reasons are dismissed and seen as misguided.
Sikhs have extensive "hair apologetics" sources in which elaborate defenses have been constructed for maintaining uncut or untrimmed hair. Keeping of the hair unaltered is seen as a sign that one has accepted their body as perfectly designed by the creator. Hair is seen to posses supernatural or spiritual powers as well as practical ones. Hair apologetics have been developed by Sikh scholars mainly to tackle the issue of Sikh men trimming their beards or cutting their hair.
- 'The hair on the head shields the person from the mind energies of other people': no evidence exists of "mind energy", so this benefit is unproven.
- 'Spiritual energies or vibrations present in hair whilst meditating': this is again another claim which needs evidence for spiritual "energies"
- 'More hair on your head means you are capable of generating more spiritual energy': again, "spiritual energy" is not well proven... Futhermore, if it were true, this would merely be a guarantor that balding men will have the least spiritual energy producing capabilities vs their more hairier brethren. Though in jest, one can posit that this as a disadvantage that men have over women as women have inferior UV shielding due to their lack of facial hair- a compromise. Nonetheless some men bald more than other men- hairy discrimination by the divine against those who have hair follicles that are more sensitive to Dihydrogen testosterone (DHT, a contributing factor to baldness).
- 'Top knot of long hair can absorb more sun energy': how this can be a benefit especially for Sikhs, is not clear considering that Sikhs are prohibited from uncovering their hair in public.
- 'Facial hair absorbs UV radiation therefore prevents radiation dose': this may be somewhat true but UV radiation is also used in vitamin D production. Vitamin D deficiency is cited as a reason to keep long hair by Sikh "hair apologists" because they believe vitamin D is produced in hair (little to no evidence of this in humans). Furthermore, this protection from UV radiation via facial hair isn't provided for females? As Sikhs are mostly of Indian rather than European ancestry, they are less likely to experience over exposure to UV rays.
- 'To distinguish between male and female': in this case it would be more appropriate to design a distinguishing feature that doesn't give one gender more of a purported advantage (i.e. UV shielding provided by facial hair) than the other. Furthermore, as the universe and all creatures are made perfect, there would be no ambiguities regarding features that are distinguishing between genders as hormonal imbalances may cause women to grow facial hair.
- 'Hair on body regulates body temperature (hair on the head as an insulator from heat loss)': though this is somewhat true, clothing appears to be a more significant and practical solution: thicker or thinner clothing.
- 'Act as a "storehouse" of trace elements such as lead, zinc, arsenic and chromium': the claim made here is that strands of hair can be used for diagnostic purposes as the hair can behave as the equivalent of tree rings in dendrochronology where the tree rings can act as a historical record. This is probably one of the very few valid points that have been made regarding keeping hair.
- 'Bald men would pay anything to have a full head of hair': most likely these men would like to have increased volume of hair but this doesn't necessarily mean that the balding men would want LONGER hair.
- 'Many Sikhs died to protect their hair, they would rather be killed than cut they hair so you shouldn't cut your hair': this argument is analogous to Christians using the "Jesus died for you" argument where an emotional argument is used. Killing people who cut their hair also constitute a violation of their rights, does that mean that if people were killed for cutting their hair that therefore people SHOULD cut their hair in honor of those killed? Or should they rather honor the right that people can and should do what they wish with their own hair or bodies.
- 'You don't cut off your arms so why cut off your hair?': hair can be regrown, limbs cannot. Limbs are useful in everyday activities, hair are not.
- 'Eye lashes, nostril and ear hairs to keep out fine dust particles': there is some truth to this and one of the very few demonstrable but not very strong justifications.
- 'Hair helps synthesizing vitamin D from sunlight': there is little evidence to suggest that humans synthesize their vitamin D via their hair. Humans produce vitamin D predominantly in their skin. Furthermore, due to the lack of Sun in northern hemispheres, we would expect people in those regions to be hairier so they may capitalize from the sunlight if vitamin D is indeed produced in hair. There is also a contradiction present here where it is claimed that hair synthesizes vitamin D but also prevents UV from reaching the skin which is KNOWN to produce vitamin D upon UV exposure.
- 'Here are some religious figures, they may have been other religious but they still had long hair; Jesus, Moohammad': many of these religious icons had unfavorable characteristics which we wouldn't adopt today simply because of their status in religious mythologies.
- 'Samson had lots of strength because of his hair!': there does not appear to be any strong evidence or correlation that cutting hair makes a person significantly weaker, otherwise the world's strongest men would all have untrimmed hair.
- 'Cutting of hair consumes a vast amount of time': this is true, but it's also true that maintaining and cleaning long hair also consumes vast amounts of time and money (costs due to increased use in oils, conditioners, shampoo, etc). Though it is a valid point that people spend an inordinate amount of time and money cutting their hair, the same can be said of those who keep long hair though their main focus is in cleaning and maintenance.
- ↑ 
- ↑ Why homeless Britons are turning to the Sikh community for food, BBC, 22 February 2015
- ↑ 
- ↑ 4.0 4.1 
- ↑ 
- ↑ 
Sikh Rehat Maryada (The Code of Sikh Conduct & Conventions). (2006). Shiromani Gurdwara Prabandhak Comittee. Ludhiana (India): Sikh Missionary College.