St. Cornelius was elected to succeed St. Fabian as the Bishop of Rome (pope), after Fabian was killed in a persecution in the year 250. The Church faced not only persecution, but also opposition from within. A priest named Novatian denied the Church's authority to forgive serious sins, such as apostasy (abandoning the faith during a time of danger). Novatian even had himself consecrated as a rival bishop of Rome, thereby becoming an anti-pope.
Pope Cornelius, backed by St. Cyprian and other bishops, upheld the Church's teaching, and allowed sinners to do penance and return to the Church. In 253, St. Cornelius was exiled by the authorities, and died a martyr soon afterward. His friend St. Cyprian wrote of Cornelius' gentle and forgiving manner.
Cyprian was a famous lawyer and orator in North Africa. He didn't become a Christian until age forty-six, then later he received the sacrament of Holy Orders, and soon became bishop of Carthage. Cyprian went into hiding during the Roman persecution of 250, to allow himself to continue ministering to his people. During this time another priest usurped his position and then forgave all apostates without requiring any penance of them at all. This position was too lenient, and Cyprian succeeded in having it condemned by the Church.
Though Cyprian was gentle and forgiving, like Cornelius, he could also be stern and uncompromising. He was executed during a persecution in 258, and St. Augustine later wrote that St. Cyprian atoned for his frequent anger and impatience by his glorious martyrdom.