Apostasy (Greek apostasia, “insurrection”), the total abandonment of Christianity by a baptized person. In the early church it was considered one of the three unpardonable sins, with the other two being murder and fornication. Apostasy is to be distinguished from laxity in the practice of religion and from heresy, the formal denial of one or more doctrines of the Christian faith. In Roman Catholic canon law, the term also refers to the abandonment of the religious state by a monk or nun who has taken perpetual vows and leaves the religious life without the appropriate dispensation. The word apostasy also can be used to describe the rejection of any religious faith.
Novatian (200?-258?), Roman theologian, who became the second antipope in 251. A leader among the Roman clergy, Novatian espoused a rigorism in church discipline that was akin to Montanism.
After the martyrdom of Pope Fabian in 250 during the persecutions of Emperor Decius, the Roman church postponed electing a successor. In 251 the church elected Cornelius as pope. Cornelius advocated the forgiveness and readmittance of Christians who had committed apostasy under persecution. Novatian, however, believed that after baptism there could be no forgiveness for grave sins. He had himself consecrated pope by three bishops from southern Italy and went into schism with his followers; in 251 they were excommunicated by Cornelius. The Novatianists established their own church, which endured until they were formally reunited with the Catholic church by the Council of Nicaea in 325. Novatian himself is thought to have been martyred during the persecutions of the Roman emperor Valerian.
Novatian was the first Roman theologian to write in Latin. Two of his nine known treatises have survived: On the Trinity and On Jewish Foods.