(or "On Penance")
by St. Ambrose
Ambrose wrote On Repentance against the Novatian heresy, which took its name from a priest named Novatian, who lived in Carthage. He was consecrated as pope by three bishops in 251 becoming an antipope. He had adopted a more rigorous position than the established Pope Cornelius in how to deal with lapsed Christians who had returned to the Church following persecution. The Church upheld and maintained it’s authority to admit any guilty party to repentance, no matter the sin, while the heresy Novatianism refused readmission to communion of the lapsi, as they were called. Novatian was thus excommunicated, but the schism persisted for several centuries.
Source Used: Nicene and Post-Nicene Fathers, Second Series, Vol. 10. Translated by H. de Romestin, E. de Romestin and H.T.F. Duckworth. Edited by Philip Schaff and Henry Wace. (Buffalo, NY: Christian Literature Publishing Co., 1896.)