To be an early martyr is to be a saint; to be an early pope and an early martyr, well that really says something. Unfortunately we are not really sure what it says. We know much about the life and pontificate of Callistus I but also unfortunately it comes from his number one enemy and rival, Hippolytus, a saint in his own right.
Callistus seems to have started out poorly as a slave, an embezzler, and convicted felon but, depending on if you believe Hippolytus or not, somehow ingratiated himself with his old master and won his freedom.
Regardless of what Hippolytus says, if Callistus was a con man then he seems to have been more of a Robin Hood type of villain, or at least he seemed to want to do good, even if with poor choices for the means. While Hippolytus tells us everything in a bad light, most of the things he tells seem good. You can put it all down to sour grapes and recognize that even saints are sinners - on both sides of the pen.
In the end, it is his education and his deep knowledge of the Faith which places him along side pope Zephyrinus and ensures his own rise to the papacy. There, perhaps because of his own failures and suffering, he led us as Church toward forgiveness of those who fail, even those who fail spectacularly - a grace that defines us to this day.
Callistus reminds us that even the worst of sinners have been redeemed but that they only become saints if they seek and live for Christ. We have benefited from his wisdom and guidance in the Seat of Peter; let us therefore strive to be like him even in the face of our enemies.
So I exhort the presbyters among you, as a fellow presbyter and witness to the sufferings of Christ and one who has a share in the glory to be revealed. Tend the flock of God in your midst, [overseeing] not by constraint but willingly, as God would have it, not for shameful profit but eagerly. Do not lord it over those assigned to you, but be examples to the flock. And when the chief Shepherd is revealed, you will receive the unfading crown of glory
-- 1 Peter 5:1-4