Blasphemy is irreverent speech or action regarding something held to be sacred. What is considered blasphemous and how (or if) it should be punished varies widely with different religious traditions. Blasphemy is sometimes used to persecute minority groups within a community, sometimes by extra-judicial punishment or murder. 
What Constitutes blasphemy?
Depending on the particular denomination, most religions do not consider phrases such as "for God's sake" or "God damn" blasphemous. Some only consider them blasphemous if they are not meant literally; i.e. "God damn" just as an expression and not God actually damning something. Others see this as taking God's name in vain, and thus blasphemy.
Islam typically does not consider such expressions to be blasphemous unless they refer to something other than the Islamic god as being divine, i.e. "Jesus Christ!" However, speaking negatively of God or prophets is considered blasphemous.
According to Leviticus 24:16 , blasphemers "shall surely be put to death."
Luke 12:10 describes blasphemy as unforgivable. However, interpretations of this passage vary widely as to what constitutes blasphemy against the Holy Spirit and whether it can be forgiven. The Catholic Church, for example, has prayers specifically for forgiveness of blasphemy.
"Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."
The freedom of speech is also not generally recognised and instead the concept of freedom from religious defamation is promoted, which is an abridgement of freedom of speech. Many Muslims think that criticism of Islam or Muhammad should not be permitted. Support for this is 58% among US Muslims,  and 62% of British Muslims. 
Islamic tradition holds that blasphemers will not enter heaven unless they repent before they die.
- Main Article: Blasphemy laws by country
In the United States, blasphemy is protected speech under the First Amendment. However, prior to the Fourteenth Amendment, the Bill of Rights was not considered to apply to state governments, and several states had laws against blasphemy on their books. These laws have been ruled unconstitutional.
The United Kingdom had blasphemy laws on the books until 2008. They specifically dealt with blasphemy against Christianity.
In Pakistan, blasphemy is punishable by life imprisonment.
Religious defamation, sometimes called the "vilification of religion", is a reframing of anti-blasphemy as a human right. On this basis, criticism of religion is curtailed in the name of "religious tolerance".
"freedom of expression without limits or restrictions would lead to [the] violation and abuse of religious and ideological rights [... everyone must] intensify efforts to criminalise insulting heavenly religions, prophets, holy books, religious symbols and places of worship"
Motions have been passed by various UN bodies that call for recognition of religious defamation. Since 2008, motions calling for curbs of religious defamation were passed by the UN general assembly but later motions gained less support with the majority of countries abstaining completely. UN resolutions encourage similar laws to be enacted around the world. This is effectively a global anti-blasphemy policy.
"defamation of religions is among the causes of social disharmony and instability, at the national and international levels, and leads to violations of human rights [...] further expresses deep concern at the intensification of the campaign of defamation of religions [...] Also urges States to provide, within their respective legal and constitutional systems, adequate protection against acts of hatred, discrimination, intimidation and coercion resulting from the defamation of any religion [...] Emphasizes that respect of religions and their protection from contempt is an essential element conducive for the exercise by all of the right to freedom of thought, conscience and religion; [...] everyone has the right to freedom of expression, and that the exercise of this right carries with it special duties and responsibilities, and may therefore be subject to certain restrictions, but only those provided by law and necessary for the respect of the rights or reputations of others, or for the protection of national security or of public order, or of public health or morals;"
- — Resolution 7/19. Combating defamation of religions, 27 March 2008
The 7/19 resolution is clearly aimed to curb freedom of speech, including on the vague grounds of "public morals" and "national security". Other religious leaders have expressed similar sentiments:
"You cannot provoke. You cannot insult the faith of others. You cannot make fun of the faith of others."
- "freedom of speech and freedom of expression do not constitute a freedom to dismiss or disrespect the beauty of anyone’s race, the sacredness of anyone’s religious belief or the uniqueness of anyone’s nationality "
Protection from defamation of belief is not a human right
Human rights are protections of vulnerable individual people against oppression. The oppression usually is usually perpetrated by more powerful individuals and groups. In practice, religious defamation is used to "defend" already powerful social groups (or even the abstract notion of "religion" ) against critics that are weak or members of minority groups. For this reason, religious defamation is not a right that is similar to other human rights.
Similar to Blasphemy laws, religious defamation laws abridge freedom of speech. They say certain topics may not be discussed. Free speech is typically regarded as fundamental to an open society, while religious defamation is not. The law supposes that religious groups need protection by not being subjected to certain ideas or criticism. While people should be protected from persecution, being subjected to criticism or other ideas is not itself harassment. Harassment and having to tolerate criticism are separate issues and should not be confused.
"Nobody has the right to not be offended. That right doesn't exist in any declaration I have ever read."
- "[Religious defamation] also still erroneously conflates blasphemy or criticism of religious ideas with incitement to acts of discrimination or violence against individuals. "
Religious toleration laws can be written without violating freedom of speech. Protection from harassment, particularly if it is religiously motivated harassment, is already a widely recognised human right.
There are many instances of religious defamation being used to suppress and intimidate political dissidents, minority denominations and religious reformers. The law itself enables and legitimises other serious human rights violations. Therefore, it is counter productive for human rights in general. Recent proposals to the UN have substituted "religious deformation" with "hate speech" but when hate speech is broadly defined it still amounts to a blasphemy law.
In 2011, the UN Human Rights Committee adopted a motion that took the opposite position.  This shows that religious defamation is not a human right, or that it has wide support.
"Prohibitions of displays of lack of respect for a religion or other belief system, including blasphemy laws, are incompatible with the [International Covenant on Civil and Political Rights], except in the specific circumstances [of incitement to discrimination, hostility or violence]. [...] Nor would it be permissible for such prohibitions to be used to prevent or punish criticism of religious leaders or commentary on religious doctrine and tenets of faith."
- — Human Rights Committee, General comment No. 34, 11-29 July 2011
The value of criticising religion
"The freedom to speak one's mind is not only an aspect of individual liberty -- and thus a good unto itself -- but also is essential to the common quest for truth and the vitality of society as a whole."
- — Chief Justice Rehnquist 
- "Historically, criticism of religion has been a crucial aspect of free expression and intrinsically linked with anti-clericalism and the dismantling of that which is deemed taboo and sacred by the gatekeepers of power. Such criticism has been key for social progress. "