When Emperor Decius began is persecution of Christians in the Roman Empire, there was still hope for Christians as the enforcement had been spotty. That all changed in the year 250, when the order was given that a Christian must perform sacrifices to the Roman gods with witnesses or be scheduled for execution. While many Christians refused and accepted the martyrdom (including Pope Fabian) others performed the sacrifices. For 14 months after the execution of Pope Fabian, there was no officially Pope and instead the Church was governed by a college of priests.
When Emperor Decius left Rome to fight the Gothic invasion, the college of priests elected Cornelius as Pope. A priest in the diocese of Rome, Novatian, was widely thought to be the leading contender, but the college went with the more moderate Cornelius and Saint Cyprian would write:
“Cornelius was elected Pope by the judgment of God and of Christ, by the testimony of most of the clergy, by the vote of the people, with the consent of aged priests and of good men.”
With this backdrop, the situation Pope Cornelius found himself was a fragmented church in a pivitol time. Novatian lead one camp believing that the lapsi (fallen) who had performed the sacrifices had committed an unpardonable sin that could only be rectified in Heaven, while Pope Cornelius and his good friend Saint Cyprian believed that the sin could be forgiven on Earth through penance. Things came to a head when a group of priests elected Novatian to the Papacy causing a split. Pope Cornelius convened a Synod of 60 bishops who acknowledge him as the rightful pope. The Synod also excommunicated Novatian and agreed that through penance the lapsi could be forgiven and fully re-admitted into full communion with the Church.
Following Emperor Decius' death in battle with the Goths, the new Emperor Trebonianus Gallus exiled Cornelius to Centumcellae Italy. During his time in Centumcellae he sent several letters to the fellow bishops, one of which is the first written example of an official office of Exoricst in the church. He would die in June of the year 253 with conflicting reports saying that he was either beheaded or died due to the hardships of banishment.
His body was buried in a catacomb near the Chapel of the Popes with the Latin inscription "Cornelius Martyr". In the middle ages, he was venerated as one of the Four Holy Marshals and today is the patron saint of earaches, fevers, cattle and the town of Kornelimunster, Germany..