XIV.
But if they say, How can there be three Persons, and how but one Divinity?—we shall make this reply: That there are indeed three persons, inasmuch as there is one person of God the Father, and one of the Lord the Son, and one of the Holy Spirit; and yet that there is but one divinity, inasmuch as the Son is the Image of God the Father, who is One,—that is, He is God of God; and in like manner the Spirit is called the Spirit of God, and that, too, of nature according to the very substance, and not according to simple participation of God. And there is one substance in the Trinity, which does not subsist also in the case of objects that are made; for there is not one substance in God and in the things that are made, because none of these is in substance God. Nor, indeed, is the Lord one of these according to substance, but there is one Lord the Son, and one Holy Spirit; and we speak also of one Divinity, and one Lordship, and one Sanctity in the Trinity; because the Father is the Cause of the Lord, having begotten Him eternally, and the Lord is the Prototype of the Spirit. For thus the Father is Lord, and the Son also is God; and of God it is said that “God is a Spirit.”
Reproduced in full. Translation: Ante-Nicene Fathers, vol. 6 (Roberts and Donaldson, 1886), public domain (Public domain).