We know precious little about St. Hippolytus (b. 170—d. 235), whose feast day is today. But one fact is certain: he was the first anti-pope in the history of the Catholic Church, thus making him a most unlikely candidate for sainthood after his death. Here’s how it happened: St. Hippolytus, accused—falsely so, according to the Catholic Encyclopedia—Pope Calixtus of advocating a particular heresy that diminished the differences between two persons of the Trinity, God the Father and God the Son. Hippolytus was, apparently, also “scandalized when Calixtus … took measures to extend absolution to graver sins such as adultery,” according to the Encyclopedia Britannica. So it was out of misplaced zeal and misunderstanding that Hippolytus had himself elected anti-pope, itself quite a serious matter. The schism lasted through several popes.
Eventually, Hippolytus was exiled to Sardinia in a period of persecution during the pre-Constantine era. The circumstances of his death are unclear, although three seems to be agreement on at least two key points: he died as a martyr and, before his death, he became reconciled with Rome. This account here says he was reconciled through the legitimate Pope, Pontian, with whom he was exiled. Both men, the site says, perished amid the harsh conditions of working the Sardinian mines. But Hippolytus is commonly depicted in art (click here for an example) as being martyred through dismemberment by horses (his name means “unleasher of horses”). Either way he came back home to Rome.
Setting aside the unfortunate matter of his schism, we know enough about Hippolytus to know that he was an important theologian, but not enough to know why in as much detail as we should like, according to the Catholic Encyclopedia:
Hippolytus was the most important theologian and the most prolific religious writer of the Roman Church in the pre-Constantinian era. Nevertheless the fate of his copious literary remains has been unfortunate. Most of his works have been lost or are known only through scattered fragments, while much has survived only in old translations into Oriental and Slavic languages; other writings are freely interpolated. The fact that the author wrote in Greek made it inevitable that later, when that language was no longer understood in Rome, the Romans lost interest in his writings, while in the East they were read long after and made the author famous. His works deal with several branches of theology. … His exegetical treatises were numerous: he wrote commentaries on several books of the Old and New Testaments. Most of these are extant only in fragments.