16 September - Cornelius became the twenty-first pope and succeeded Fabian, 14 months after the latter's death on January 20, 250. After Fabian's death, the persecution of Emperor Decius was so violent that Christians in Rome had to wait more than a year to elect a new bishop.

After Fabian's death, the persecution of Emperor Decius was so violent that the Christians of Rome had to wait more than a year to elect a new bishop. In this difficult context, the administrative organization of the Church, put in place by Fabian, proved its effectiveness and allowed for collective decision-making by the various clergy. However the primacy of the Church of Rome is already affirmed and to answer the requests of the other Churches the clerics call upon Novatian. In March 251 the election takes place and surprise: it is the priest Cornelius who is elected. The reason is simple. Many Christians, during the persecution of Decius, had abjured their faith out of fear or opportunism. Many wanted to return to the Church again. Two attitudes are then opposed: the intransigent ones around Novatian, and those who follow forgiveness and succeed in having Cornelius elected. A new schism then appears because three Italian bishops agree to consecrate Novatian while almost all the other Churches recognize Cornelius. A synod, gathered in autumn 251, with Bishop Denys of Alexandria and Cyprian of Carthage, approved the leniency of Cornelius and excommunicated Novatian for his harshness towards the repentant. The patriarch Fabian of Antioch, who, like many oriental bishops, was more firm, was the recipient of a letter sent by Cornelius in which he argued his point of view (Clavis Patrum Græcorum 1850-1854). Cornelius was deported by order of Emperor Trebonian Wales to Centumcellae Civitavecchia at the end of 252, where he died, probably naturally, in 253. His body was brought back to Rome and placed in the catacomb of St. Calixtus.

Cyprian of Carthage, whose real name was Thascius Caecilius Cyprianus, was born in North Africa around 200, of pagan parents of Berber origin. He first made a career as a rhetorician in Carthage. He professed rhetoric and converted to Christianity quite late. He became a priest and then, in 249, bishop of Carthage. During the persecution of Decius, he remains far from Carthage; this "flight", for which he is blamed, aggravates the difficulties he has to resolve: revolt of the confessors, problem of the reconciliation of the lapsi, bursting of schisms on this subject in Africa and in Rome, where Novatian chooses severity and founds a dissident Church promised to a long future. The death of Decius in 251 brought him a few years of respite, despite threats of persecution and the outbreak of an epidemic. When the first edict persecuting Valerian appeared, Cyprian was exiled in August 2577; a year later, having returned to his episcopal city, he was beheaded there, by virtue of the second edict, on September 14, 258, with several of his ecclesiastical companions, including Flavian of Carthage. Saint Cyprian wrote numerous treatises and letters in Latin. Their object and purpose is to defend Christianity and to support the faith of Christians. The letters of Saint Cyprian are valuable historical documents, especially for understanding the evolution of ecclesiastical law. He left a great number of writings, among which are: "De lapsis" (From the Fallen): those who had fallen during the persecution of Decius were called so; "De Catholicae Ecclesiae unitate" (From the Unity of the Catholic Church): against those who sought to create division in the Church.