To be called a "Sikh", according to the Sikh Rehat Maryada, one must fulfill the following criteria to be: i) believes in one immortal deity ii) follows the teachings of the ten gurus iii) accepting the Guru Granth Sahib (holy book) as the final guru iv) has been baptised v) does not owe allegiance to other religions.
Most self-professed Sikhs do not fulfill this 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 is monotheistic. However, the deity is not as anthropomorphic and interventionist compared to the deity in the Abrahamic religions, though, in essence they worship the same being. The god concept in the Sikh religion is not easily discernable, and this due to the deity being seen as deistic but also pantheistic considering the great deal of pantheism ("poetic", or "sexed up atheism") in the literature. Sikhs refer to god as "Waheguru" (meaning wonderful/great teacher), but have no issue using other names to refer to god such as Allah or Raam. Further, the Sikh scripture also uses various names to describe the same being.
Sikhs congregate in a "Gurdwara" or temple (house of the guru, a Sikh version of a church or mosque). 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
Sri Guru Granth Sahib (SGGS for short): The composition of poetry from various authors, completed several hundred years ago. Though some of the authors were from Hindu or Muslim backgrounds, they shared the same worldview as the gurus (i.e. they behaved in accordance with the Sikh religion).
Sikh Rehat Maryada: Sikh code of conduct- this is generally authored by a commitee of Sikh scholars on what they feel is consistent with the teachings in the Guru Granth Sahib and was created almost a hundred years ago. The Sikh Rehat Maryada contains information about how to conduct birth, marriage and funeral ceremonies.
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. 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 acts that Sikhs must not commit, which include the following:
1) Cutting/shaving/trimming/waxing any hair be it facial, armpit, or pubic...
2) Eating meat (especially ritually slaughtered meat, i.e. halal, kosher)
3) Engaging in extramarital sex
4) Consuming drugs
5) Consumption of alcohol (alcohol is also a drug, however, the number 5 has great favour amongst Sikhs, hence the "extra" deadly sin)
Baptised 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-haves" for baptised 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 (predominantly Muslims) aggression, in which the Sikh religion began advocating the carrying of arms/weapons. The idea behind the 5Ks is for Sikhs to maintain constant/perpetual readiness for battle.
1) Untrimmed/unaltered hair (kés): long hair is seen as submission/acceptance of the deity's design.
2) Comb (kunga): to maintain the hair.
3) Bangle (kara): this is meant as both an article of faith (whenever the follower is doing something, the bangles are there to remind them of their duties, and also the "omnipresence" being symbolised by the circle) and as a defensive/offensive weapon (initially, bangles were of a different design, some of which had razor sharp edges to be used as frizbeez, and some designs provided a degree of protection for the wrist). The bangle is supposed to be made from iron/steel, not gold, and must be worn on both wrists.
4) Kirpan (sword): akin to the 2nd ammendment of the US constitution with the difference that carrying of arms is seen as mandatory. Again, contrary to what many non-Sikhs (and even some Sikhs) think, the kirpan is NOT just a symbol of faith, but also a weapon (primarily for defensive purposes).
5) 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.
Afterlife in Sikhism
Sikhism puts forward the idea that the human form is a chance to escape perpetual reincarnation by living according to the ways of Sikhi, non-Sikhs continue to undergo reincarnation. 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.
"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.
Superstition and Sikhism
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.
Sikhism and evolution
Unlike Christians and Muslims who view evolution as a possible challenge to the religion, Sikhs do not. However, this may be due to the Sikh religion having little to no mention of the Adam and Eve story on which the concept of original sin in Christianity is based. Though the Sikh religion is monotheistic, it is not an Abrahamic religion, which is why many of the objections that Christians and Muslims have towards science are not present amongst Sikhs (example: the Adam and Eve story, Noah's flood). It is however, possible for Sikhs to have objections to science, archaelogy, etc, if a challenge to any of the Sikh doctrines, stories, etc, is posed- in which case the religon takes precedence.
Who Sikhs are not allowed to have friendships/associations with
According to the Sikh Rehat Maryada, 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:
i) Those who eat or drink from an unbaptised persons plate
ii) One who dyes their hair
iii) Sikhs who previously had uncut hair but have now cut it
iv) Those who have used drugs (i.e. alcohol, marijuana, cocaine)
v) Anyone who generally engages in practises which contradict the teaching of the gurus
Interfaith relations in Sikhism
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 emnity 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 could be encouraged considering the views these religions hold which are criticised in Sikhism, has not been explained. 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.
According to the Sikh Rehat Maryada, Sikhs are not permitted to marry non-Sikhs and special emphasis is placed on Sikh women marrying non-Sikh males. Despite this, Sikhs marrying non-Sikhs does 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.
Though the Sikh scriptures are not inherently violent (as is the Bible and Qu'ran), a few questionable verses exist which need further examination:
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.
Apostasy in Sikhism
There is no penalty for leaving the Sikh religion (no violence is advocated in the Sikh literature against those who leave the faith), though those that openly claim to have left the faith tend to be ostracized in the more religiously devout communities. 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. Though punishment for apostasy does not officially exist in Sikhism, it is not unheard of for baptised or devout Sikhs to take the harsher route in which those who have left the faith are evicted from their families homes.
Sikhism and "proofs of god"
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(example 1,example 2). 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.
Sikh Rehat Maryada (The Code of Sikh Conduct & Conventions). (2006). Shiromani Gurdwara Prabandhak Comittee. Ludhiana (India): Sikh Missionary College.