Quintus Septimius Florens Tertullianus (c.155–c.222), who was converted to Christianity before 197, was a prolific writer of apologetic, theological, controversial and ascetic works. He was born in Carthage, the son of a Roman centurion.
Tertullian denounced Christian doctrines he considered heretical, but later in life adopted the Montanist religion, regarded as a heretical sect by the mainstream Church. He wrote three books in Greek and was the first great writer of Latin Christianity, thus sometimes known as the "Father of the Latin Church". He was evidently a lawyer in Rome. He is said to have introduced the Latin term "trinitas" with regard to the Divine (Trinity) to the Christian vocabulary (but Theophilus of Antioch already wrote of "the Trinity, of God, and His Word, and His wisdom", which is similar but not identical to the Trinitarian wording), and also probably the formula "three Persons, one Substance" as the Latin "tres Personae, una Substantia" (itself from the Koine Greek "τρεῖς ὑποστάσεις, ὁμοούσιος; treis Hypostases, Homoousios"), and also the terms "vetus testamentum" (Old Testament) and "novum testamentum" (New Testament).
In his Apologeticus, he was the first Latin author who qualified Christianity as the "vera religio", and systematically relegated the classical Roman Empire religion and other accepted cults to the position of mere "superstitions".
Later in life, Tertullian joined the Montanists, a heretical sect that appealed to his rigorism. He used the early church's symbol for fish—the Greek word for "fish" being ΙΧΘΥΣ which is an acronym for "Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ" (Jesus Christ, God's Son, Saviour)—to explain the meaning of Baptism since fish are born in water. He wrote that human beings are like little fish.