Tertullian, Quintus Septimus Florens (ad c. 160-c. 240) Roman Christian polemicist. Born in Carthage, Tertullian converted to Christianity around 196, after which he wrote a string of fierce essays defending Christianity, attacking Platonism and Hellenizing tendencies, and expounding an ethic of martyrdom. Heresy comes from trusting philosophy rather than faith. Tertullian joined a rigorous sect, the Montanists, but seems to have left to promote a sterner asceticism than theirs.