Quintus Septimus Florens Tertullianus, living circa 160 - 230 AD, was an early influential Christian writer and theologian. Tertullian converted to Christianity in 197 AD and, having an excellent education in law, became a prominent defender of the faith and victims of persecution. As prominent as he was, Tertullian split from the early Catholic Church, however, joining the Montanism movement in 213 AD.
Unlike the Catholics, the Montanists were strict believers in the concept of not forgiving fallen Christians, while the Catholics believed in the ability to repent and be re-accepted by the Church. Montanists also encouraged martyrdom as a means to spread the faith and believed that they lived within the time of the apocalypse and impending judgement. Because of his split, Tertullian was never recognized as a saint within the Catholic Church, despite his immense contribution to its defense and spread.