Tertullian
Though conservative in his worldview, Tertullian originated new theological concepts and advanced the development of early Church doctrine. He is perhaps most famous for being the first writer in Latin known to use the term trinity (Latin: trinitas). According to The Stanford Encyclopedia of Philosophy, "Tertullian's trinity [is] not a triune God, but rather a triad or group of three, with God as the founding member".[13] A similar word had been used earlier in Greek,[a] though Tertullian gives the oldest extant use of the terminology as later incorporated into the Nicene Creed at the Second Ecumenical Council, the First Council of Constantinople in 381 AD, or as the Athanasian Creed, or both.[14] Other Latin formulations that first appear in his work are "One God, three persons, one substance" as the Latin "tres personae, una substantia", ('consubstantial', in English), itself from the Koine Greek "treis hypostases, homoousioi").[12] Influenced by Stoic philosophy, the "substance" of Tertullian, however, was a material substance that did not refer to a single God, but to the sharing of the divine substance of the Father, the Son, and the Holy Spirit.[13] He wrote his understanding of the three members of the trinity after becoming a Montanist.[12]