Apostasy

From the Greek apo, "away from," and stasis "standing," literally meaning a "standing apart," apostasy is used in Christian theology to speak of total renunciation of faith in Christ and abandonment of Christianity. It has always been considered to be among the most serious sins. Apostasy was regarded as unforgivable in the post-apostolic church. However, the magnitude of defections during the Decian persecutions in 250 compelled the church to apply its gospel of forgiveness to apostates. Hippolytus and Tertullian were rigorists in the discussion, but Callistus advocated a more merciful course. Cyprian required rebaptism. 

 
The Diocletian persecutions in 303 spawned the Donatist controversy. Donatists claimed that only the pure could remain in communion with the church. Optatus and Augustine framed the catholic response. They urged that the church is a societas mixta, saints and sinners mixed together, and that even apostates could be accepted back into the communion of the church after rigorous penance. They did not demand rebaptism. Their position continues to be upheld by the church. 
 
One who commits apostasy is an apostate. See Abjuration. 
 

Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.