early 3rd century – c. 250

Tertullian's Against Praxeas and Novatian's On the Trinity: one substance in three Persons

Answering Praxeas, who, in Tertullian's words, "drove out the Paraclete and crucified the Father", Tertullian in Against Praxeas introduced the Latin trinitas and the formula: the Father, the Son and the Spirit are three distinct but undivided Persons (persona) of one substance (substantia). To Praxeas's "monarchy" he opposed the "economy" (dispensatio, oikonomia), explaining it with the images of root, stem and leaves, sun and ray, spring, river and canal. The books agree that his terms entered the later definitions of the Trinity and the incarnation, although Berkhof and Lane note that the formula was not carried through because of the subordination of the Son. Around 250 the Roman presbyter Novatian, in his treatise On the Trinity, following Tertullian, argued against Sabellianism for the deity of the Son and his distinction from the Father, calling the Son "the second Person after the Father".