<P> 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 Ιησοῦς 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 . </P> <P> Saint Cyprian (Thascius Caecilius Cyprianus) (died September 14, 258) was bishop of Carthage and an important early Christian writer . He was born in North Africa, probably at the beginning of the 3rd century, perhaps at Carthage, where he received an excellent classical (pagan) education . After converting to Christianity, he became a bishop and eventually died a martyr at Carthage . He emphasized the necessity of the unity of Christians with their bishops, and also the authority of the Roman See, which he claimed was the source of "priestly unity"' . </P> <P> Hilary of Poitiers (c. 300--c. 368) was Bishop of Poitiers and is a Doctor of the Church . He was sometimes referred to as the "Hammer of the Arians" (Latin: Malleus Arianorum) and the "Athanasius of the West ." His name comes from the Greek word for happy or cheerful . His optional memorial in the Roman Catholic calendar of saints is 13 January . In the past, when this date was occupied by the Octave Day of the Epiphany, his feast day was moved to 14 January . </P> <P> Saint Ambrose was an archbishop of Milan who became one of the most influential ecclesiastical figures of the 4th century . He is counted as one of the four original doctors of the Church . He offered a new perspective on the theory of atonement . </P>

Difference between fathers and doctors of the church