3 But all this seems absurd to those who drag up familiar things to a level with things little known, visible things with invisible, and compare the creature to the Creator. For unbelievers sometimes question us and say: Whom you call the Father, do you call him God? We answer, God. Whom you call the Son, do you call him God? We answer, God. Whom you call the Holy Spirit, do you call him God? We answer, God. Then, say they, are the Father, and the Son, and the Holy Spirit three Gods?
We answer, No. They are confounded, because they are not enlightened; they have their heart shut up, because they want the key of faith. Let us then, brethren, by an antecedent faith that heals the eye of our heart, receive without obscurity what we understand—and what we understand not, believe without hesitation; let us not quit the foundation of faith in order to reach the summit of perfection. The Father is God, the Son is God, the Holy Spirit is God: and yet He is not the Father who is the Son, nor He the Son who is the Father, and the Holy Spirit, the Spirit of the Father and the Son, is neither the Father nor the Son.
The Trinity is one God. The Trinity is one eternity, one power, one majesty—three, [but not three] Gods. Let not the reviler answer me: “Three what, then? For,” he adds, “if there are three, you must say, three what?” I reply: The Father, and the Son, and the Holy Spirit. “See,” he says, “you have named three; but express what the three are?” Nay, count them yourself; for I make out three when I say, the Father, and the Son, and the Holy Spirit. For the Father is God as respects Himself, but [He is] the Father as respects the Son; the Son is God as respects Himself, but He is the Son as regards the Father.
Source: Tractates on the Gospel of John (New Advent)