Apostasy is the abandonment of a religion by one of its own adherents. It is distinguished from heresy, which is holding erroneous belief while remaining within, and from schism, which is breaking communion while retaining the faith. Nearly every tradition has a category for it; what differs enormously is what follows.
Judaism uses meshummad for one who has abandoned the covenant, but the governing halakhic principle is that a Jew who apostatises remains a Jew, so that the status is not lost even when the community's obligations toward the person are curtailed. The disabilities imposed have been communal and civil rather than capital in any modern practice, and the biblical penalties concerning enticement to idolatry (Deuteronomy 13) were surrounded by rabbinic procedural requirements, witnesses, warning, courts of a kind no longer constituted, that made their application effectively impossible, in line with the rabbis' recorded discomfort with capital punishment generally.
Christianity treated apostasy severely once it had the power to do so. The early church's bitter disputes over the lapsi after the Decian persecution, whether those who had sacrificed under compulsion could be readmitted, produced the Novatianist schism and, eventually, the penitential system itself. Medieval Christendom prosecuted apostasy and heresy through ecclesiastical courts with sentences executed by secular authorities. Modern Christian bodies impose only spiritual and communal penalties, and the Catholic Church's declaration Dignitatis Humanae (1965) affirmed religious freedom as a civil right that no one may be coerced against.
Islam is where the question is most consequential today. Classical jurisprudence in all the Sunni schools and in Twelver Shia law prescribed death for the adult male apostate who does not recant within a stated period, with variations for women, imprisonment until repentance in the Hanafi school. The rulings rest on hadith, not on the Qur'an, which condemns apostasy repeatedly and at length while specifying no worldly punishment for it, and which states that "there is no compulsion in religion" (2:256). A substantial body of modern Muslim scholarship, including Taha Jabir al-Alwani, Jamal Badawi, Abdullah Saeed and, in qualified form, others, argues that the classical ruling addressed apostasy as treason and desertion in a setting where religious and political allegiance were the same thing, and does not reach private change of belief. Other scholars maintain the classical position, and the argument within Muslim scholarship is unresolved.
In practice, a small number of states retain capital punishment for apostasy in law, and executions under it are rare; but prosecutions, blasphemy charges used as proxies, loss of custody and inheritance, family coercion and severe social sanction affect a far larger number of people than formal law does, and the same is true in some non-Muslim contexts for those leaving high-demand religious groups. The freedom to change one's religion is protected by Article 18 of the Universal Declaration of Human Rights and by Article 18 of the International Covenant on Civil and Political Rights, and it remains one of the sharpest points of contention between international human rights instruments and the domestic law of several states.