Tertullian (160 - 225 A.D.) grew up as a pagan in Carthage, North Africa until converting to Christianity. Little is known of his life, but many of his writings are known to us today and are rich in information into early Christian thought and history. He condemned the heretic Marcion and wrote an Apologetic trying to persuade secular authorities to be tolerant of Christianity (i.e.— Allow it to exist under the law).
Some time before A.D. 210 Tertullian left the orthodox church to join a new sectarian movement known as Montanism (named for its founder, a self-proclaimed prophet, Montanus). Tertullian had grown angry at what looked like compromise creeping into the church—unwillingness to be martyred, willingness to forgive more serious public sins—and aligned himself with the Montanists. It is unclear whether this involved actually leaving the church, but his later works are avowedly Montanist, and one or two explictly attack the mainstream church on these points. As such he was not recognised as a Saint, despite his theological orthodoxy, and his works were all marked as condemned in the 6th Century Decretum Gelasianum (Decree of Pope Gelasius).