Hippolytus of Rome

Hippolytus is awkward — he was the first antipope, set up against Pope Callixtus around 217 in a dispute over discipline and theology, and reconciled with the church only when both he and his rival were exiled to the Sardinian mines and died there. But his writings are gold. His Apostolic Tradition is the earliest detailed description of how a Roman Christian community actually worshipped — the eucharistic prayer, baptism, ordination — and modern liturgies (Catholic, Anglican, Lutheran) drew on it heavily in the twentieth-century reforms. His Refutation of All Heresies preserves quotations from Gnostic teachers we'd otherwise have lost. The first schismatic in Rome is also one of our best windows into the third-century church.