This declaration may be made if the alleged Muslim declares himself a kafir, but more typically applies to a judgement that an action or statement by the alleged Muslim indicates his knowing abandonment of Islam.
The sentence for apostasy (irtidad) under Sharia is traditionally interpreted as execution but alternately might be amputation or expulsion.
For this reason orthodox Islamic law normally requires extremely stringent evidence for such accusations.
In many cases an Islamic court or a religious leader, an alim must pronounce a fatwa (legal judgement) of takfir against an individual or group.
There are disputes among different schools of religious thought as to what constitutes sufficient justification for declaring takfir.
The orthodox Sunni position is that sins generally do not prove that someone is not a Muslim, but denials of fundamental religious principles do. Thus a murderer, for instance, may still be a Muslim, but someone who denies that murder is a sin is a kafir if he is aware that murder is considered a sin in Islam.
An extreme case is exemplified by the early Kharijites, some of whom concluded that any Muslim who sinned ceased to be a Muslim, while others concluded that any major sin could cause that. The opposite extreme was taken by the Murjites, who argued that anyone who called themselves Muslim should be considered Muslim.
The Mu'tazilites (followed by the Zaydis) advocated what they saw as a middle way, whereby grave sinners were categorized neither as believers nor as kafirs.
Some Muslims consider takfir (declaring someone a kafir) to be a prerogative of either the Prophet—who does that through Divine revelation—or the State which represents the collectivity of the Ummah (the whole Muslim community).[1]
Extremist movements that practice takfir—both the early medieval Kharijites and modern groups such as Takfir wal-Hijra and GIA— usually regard virtually all self-styled Muslims as kafirs. As such their blood may legitimately be shed. These groups have been condemned by more mainstream Muslims.