quality:correction, linguistic:translation
Christianity

The Christian understanding of apostasy is "a willful falling away from, or rebellion against, Christian 'truth.' Apostasy is the rejection of Christ by one who has been a Christian...", but the Reformed Churches teach that, in contrast to the conditional salvation of Lutheran, Roman Catholic, Methodist, Eastern Orthodox, and Oriental Orthodox theology, salvation cannot be lost once accepted (perseverance of the saints).

"Apostasy is the antonym of conversion; it is deconversion." B. J. Oropeza states that apostasy is a "phenomenon that occurs when a religious follower or group of followers turn away from or otherwise repudiate the central beliefs and practices they once embraced in a respective religious community." The Ancient Greek noun ἀποστασία apostasia ("rebellion, abandonment, state of apostasy, defection") is found only twice in the New Testament (Acts 21:21; 2 Thessalonians 2:3). However, "the concept of apostasy is found throughout Scripture." The Dictionary of Biblical Imagery states that "There are at least four distinct images in Scripture of the concept of apostasy. All connote an intentional defection from the faith." These images are: Rebellion; Turning Away; Falling Away; Adultery.

Speaking with specific regard to apostasy in Christianity, Michael Fink writes:

Apostasy is certainly a biblical concept, but the implications of the teaching have been hotly debated. The debate has centered on the issue of apostasy and salvation. Based on the concept of God's sovereign grace, some hold that, though true believers may stray, they never totally fall away. Others affirm that any who fall away were never really saved. Though they may have "believed" for a while, they never experienced regeneration. Still others argue that the biblical warnings against apostasy are real and that believers maintain the freedom, at least potentially, to reject God's salvation.

In the recent past, in the Roman Catholic Church the word was also applied to the renunciation of monastic vows (apostasis a monachatu), and to the abandonment of the clerical profession for the life of the world (apostasis a clericatu) without necessarily amounting to a rejection of Christianity.