Tertullian

A fiercely brilliant rhetorician of Roman Carthage, Tertullian converted in adulthood and became the first great Christian author to write in Latin, effectively inventing the church's Latin vocabulary. His Apology skewered the injustice of persecution — 'the blood of the martyrs is seed' — while treatises poured out on baptism, prayer, repentance, and the errors of Marcion. He gave the West the word Trinitas and the formula of one substance in three persons. Late in life his rigorism drew him toward the Montanist movement, which is why this indispensable father, quoted ever after, was never called a saint.

He coined the Latin word Trinitas — Trinity — giving Western Christianity the very vocabulary of its central doctrine.