The term, “Trinity” was used by St. Theophilius of Antioch, in the year A.D. 180. He wrote in his epistle to Autolycus, a pagan critic of the Catholic Church, that God, His word, and wisdom are a trinity (Greek:triados).
Some year later, Tertullian, (A.D. 160—circa 250) a Latin theologian, coined the Latin term for trinity (trinitas), in his work “On Modesty.” He wrote about the trinity of the One Divinity: Father, Son, and Holy Spirit. In addition, he explained more specifically that the Father, Son, and Holy Spirit were of “One in essence—not one in Person.” (Ante-Nicene Fathers 3.621; circa 213 A.D.)
The early church writers drew upon many biblical references to God’s unity and transcendence as well as old testament episodes, known as “Theophanies”—mysterious appearances of one or more persons of the Trinity.[i]
Examples of theophanies and mysterious appearances, and sometimes implicit references to the three persons in the Trinity are Genesis 1:26, (where God speaks of Himself in the plural form), Genesis 3:22, 11:27, Psalms 2:7, 109: 1-3; Isaiah 7:14 (Emmanuel is the name prophesied for the miraculously-conceived child of a virgin. It means “God is with us”): Isaiah 9:6, 11:2, and 8:22-31, Wisdom 7: 22-28, 8: 3-8, Ezkiel 11:5, 36:27, Joel 2:28, and Malachi 3:1.