the total rejection of Christianity by a baptized person who, having at one time professed the Christian faith, publicly rejects it. It is distinguished from heresy, which is limited to the rejection of one or more Christian doctrines by one who maintains an overall adherence to Jesus Christ.

A celebrated controversy in the early church concerned sanctions against those who had committed apostasy during persecution and had then returned to the church when Christians were no longer being persecuted. The question at stake was whether the apostates should be accepted again into the church. Some early Christian emperors added civil sanctions to ecclesiastical laws regarding apostates. Certain theologians of the 4th and 5th centuries considered apostasy to be as serious as adultery and murder. In the 20th century, Roman Catholic Canon Law (Canon Law, Code of) still imposed the sanction of excommunication for those whose rejection of the faith fitted the technical definition of apostasy. But the absence of civil sanctions and an increasing tolerance of divergent viewpoints have tended increasingly to mitigate the reaction of believers to those who reject Christianity.

The term apostasy has also been used to refer to those who have abandoned the monastic and clerical states without permission.