Interestingly, the best information we have that is historically verifiable on St. Callistus comes from one of his greatest enemies: St. Hippolytus. St. Hippoltyus was an early antipope of the Church who, after his conversion and repentance, would go on to die as a martyr for the True Faith.
The following is taken from Fr. Don Miller, OFM:
St. Callistus was a slave in the imperial Roman household. Put in charge of the bank by his master, he lost the money deposited, fled, and was caught. After serving time for a while, he was released to make some attempt to recover the money. Apparently he carried his zeal too far, being arrested for brawling in a Jewish synagogue. This time he was condemned to work in the mines of Sardinia. He was released through the influence of the emperor’s mistress and lived at Anzio.
After winning his freedom, Callistus was made superintendent of the public Christian burial ground in Rome (still called the cemetery of Saint Callistus), probably the first land owned by the Church. The pope ordained him a deacon and made him his friend and adviser.
He was elected pope by a majority vote of the clergy and laity of Rome, and thereafter was bitterly attacked by the losing candidate, Saint Hippolytus, who let himself be set up as the first antipope in the history of the Church. The schism lasted about 18 years.
Hippolytus is venerated as a saint. He was banished during the persecution of 235 and was reconciled to the Church. He died from his sufferings in Sardinia. He attacked Callistus on two fronts—doctrine and discipline. Hippolytus seems to have exaggerated the distinction between Father and Son (almost making two gods) possibly because theological language had not yet been refined. He also accused Callistus of being too lenient, for reasons we may find surprising: 1) Callistus admitted to Holy Communion those who had already done public penance for murder, adultery, and fornication; 2) he held marriages between free women and slaves to be valid—contrary to Roman law; 3) he authorized the ordination of men who had been married two or three times; 4) he held that mortal sin was not a sufficient reason to depose a bishop; 5) he held to a policy of leniency toward those who had temporarily denied their faith during persecution.
Callistus was martyred during a local disturbance in Trastevere, Rome, and is the first pope (except for Peter) to be commemorated as a martyr in the earliest martyrology of the Church.St. Callistus's decree that a marriage between a woman and a slave could be valid put the Holy Father in direct conflict with Roman civil law, but he stated that in matters concerning the Church and the Sacraments, Church law trumped civil law. He taught what the Church has taught for centuries, including today, and though a whole host of schismatics wrote against him, his crime seems to have been that he practiced pure, orthodox Christianity. While he was vigorously opposed to heresy, his charitable attitude toward repentant sinners incurred the wrath of contemporary rigorists.
May St. Callistus pray for us in our world today that we have the courage to defend the unchanged teachings of the Church and the Faith even against enemies inside and outside of the Church.
Collect from his feastday on October 14th:
O God, You see that we fail because of our weakness. Be merciful to us and let the example of Your saints renew our love of You. Through our Lord . . .