4 We do not bring in two Gods as some do, who say, “God the Father and God the Son, but greater God the Father and lesser God the Son.” They both are what? Two Gods? You blush to speak it, blush to believe it. Lord God the Father, you say, and Lord God the Son: and the Son Himself says, “No man can serve two Lords.” In His family shall we be in such wise, that, like as in a great house where there is the father of a family and he has a son, so we should say, the greater Lord, the lesser Lord?
Shrink from such a thought. If you make to yourselves such like in your heart, you set up idols in the “one soul.” Utterly repel it. First believe, then understand. Now to whom God gives that when he has believed he soon understands; that is God's gift, not human frailness. Still, if you do not yet understand, believe: One God the Father, God Christ the Son of God. Both are what? One God. And how are both said to be One God? How? Do you marvel? In the Acts of the Apostles, “There was,” it says, “in the believers, one soul and one heart.” There were many souls, faith had made them one.
So many thousands of souls were there; they loved each other, and many are one: they loved God in the fire of charity, and from being many they have come to the oneness of beauty. If all those many souls the dearness of love made one soul, what must be the dearness of love in God, where is no diversity, but entire equality! If on earth and among men there could be so great charity as of so many souls to make one soul, where Father from Son, Son from Father, has been ever inseparable, could They both be other than One God? Only, those souls might be called both many souls and one soul; but God, in Whom is ineffable and highest conjunction, may be called One God, not two Gods.
Source: On the Creed: A Sermon to Catechumens (New Advent)