Today we hit Tertullian and the Trinity. You can listen here.
Tertullian is the first writer whom we know to use the word Trinity, though he uses it, and other vocabulary, as if he were saying nothing new, and were answering old questions that have been brought up anew.
Tertullian’s mind, sharp, analytical, and precise, made distinctions not previously embraced, and went beyond the Christology/Triadology of the second century by seeing that the Father, though the source of the divinity of the Logos, was not the per se divinity, and that the Son must therefore be identical in every way to the Father to be divine, the error of the Modalists and the Arians of the 4th century.
This was a fundamental insight, and while Tertullian voices this as consonant with the Tradition, what he calls the Rule of Faith, we have no writings left to us from earlier authors who have made the point as did he in his Against Paraxeas.
Below are excerpts from first his Apology (c. 190) and then his Against Praxeas (c. 213).
From The Apology
We have been taught [that the Logos] was produced (prolatum) from God (ex Deo) and in [this] production generated, and therefore is called the Son of God and God, because of the unity of the substance, since God also is Spirit. Just as when a ray is put forth (porrigitur) from the sun, it is a portion of the whole, but the sun will be in the ray, because it is a ray of the sun, and is not separated from the substance, but extended out; so Spirit [is extended] from Spirit and God from God, as light is kindled from light. The source of the material (matrix materae) remains entire and undiminished although you draw out from it many branches of its kind: thus also what is derived from God is God and the Son of God, and the two are one. In this manner, then, He who is Spirit from Spirit and God from God made another individual in mode [of existence], in grade, not in state, and did not separate from but stretched out from the source. This ray of God, then, descended into a certain virgin, as it had always been predicted in times past.
From Against Praxeas
[On the unity of the Godhead but in three Persons] As if One might not be All in this way also—viz., in All being of One, by unity of substance, while the mystery (sacramentum) of the economy is still preserved, by which the unity is distributed into a Trinity, ordering the three,—Father, Son and Holy Ghost,—three, however, not in status but in grade, not in substance but in form, not in power but in aspect (species) ; yet of one substance, and of one status, and of one power, inasmuch as there is one God from whom are reckoned these grades and forms and aspects under the name of the Father and Son and Holy Ghost.