By Water & Blood

Book 1, Chapter 11: How the Church knows one God, while receiving the Son as God from God

But if it makes them afraid that they might perhaps seem to proclaim two Gods, let them know that, even though the Son is confessed by us as God, there would be one God alone: that one who alone is without beginning and unbegotten, who possesses the Godhead as his own, and who has become for the Son himself the cause both of his being and of his being such as he is. Through him the Son himself confesses that he lives, saying plainly, “As the living Father hath sent me, and I live by the Father,” and, “For as the Father hath life in himself; so hath he given to the Son to have life in himself.”

Therefore he teaches that the Father is God both of us and of himself, where he says, “I ascend unto my Father, and your Father; and to my God, and your God.” And the great Apostle teaches that God is head of the Son himself, while the Son is head of the Church, saying in one place, “and the head of Christ is God,” and in another, concerning the Son, “and gave him to be the head over all things to the church, which is his body.”

He himself, then, would be the ruler (ἀρχηγός) and head of the Church, while the Father is his head. Thus there is one God, the Father of the only-begotten Son, and one head even of Christ himself. And since there is one origin (ἀρχή) and head, how could there be two Gods, and not that one alone who inscribes no one above himself nor any other as his own cause; who possesses as his own the unoriginate and unbegotten Godhead of the monarchic authority, and has imparted to the Son a share of his own Godhead and life; who through him brought all things into subsistence; who sends him, who commands him, who charges him, who teaches him, who delivers all things to him, who glorifies him, who exalts him on high, who has declared him King of the universe, who has delivered all judgment to him, who also wills that we obey him, who bids him occupy the throne at the right hand of his majesty, addressing him and saying,

Sit thou at my right hand;

who for all these reasons is also God of the Son himself; in obedience to whom his only-begotten Child “emptied himself, humbled himself, took upon him the form of a servant, became obedient unto death”; to whom he also prays, whom he obeys when he commands, to whom he gives thanks; whom he teaches us to regard as “the only true God,” whom he confesses to be greater than himself; and whom, beyond all this, he wishes all of us to know as his own God.

And as the Son thus glorifies his own Father, the Father in return glorifies him, and has declared him Lord and Saviour and God of the universe and sharer of the throne of his kingdom. Having been taught these things, the Church of God thinks it right to know him as her own God and Lord and Saviour, and as the only-begotten Son of the God who is over all; and she does not see fit to proclaim any other of the things that have come into being as God, but knows this one alone as God, whom alone the Father begot from himself,

just as he himself set forth through Solomon, saying, “Before the mountains were settled, and before all the hills, he begetteth me”—in this way hinting, by “mountains” and “hills,” at the divine and supramundane powers.