Pope Saint Callixtus I
There have been three popes who started off as slaves and Callixtus (d. c. 223) was the third of them. If that wasn’t a bad start to a papal career, consider that Callixtus was also condemned (probably unfairly) for embezzlement and sentenced to a life term in the mines of Sardinia. Fortunately for him, the favourite mistress of the murderous emperor Commodus (the bad guy in the movie Gladiator) was a Christian and secured his release.
After leaving the mines, Callixtus was given a pension by the pope and then a series of church positions, becoming the superintendent of the Christian cemetery just outside of Rome, still known to this day as the Catacombs of Callixtus. By 218, he had rehabilitated his reputation enough that he was elected pope. His gentle spirit and love of forgiveness made him a number of enemies who would have preferred a harsher line toward repentant sinners or schismatics rejoining the Church. Tertullian wrote against his decision to readmit to communion those fornicators or murderers who had repented. Hippolytus denounced him as a heretic and had himself elected as the first anti-pope. Legend has it that Callixtus was martyred in 223 by being thrown down a well.