Chapter VII. Peter Appeals to the Old Testament to Prove the Unity of God
And Peter said: “I shall reply briefly to what you have said. The law, which frequently speaks of gods, itself says to the Jewish multitude, 'Behold, the heaven of heavens is the Lord's your God, with all that therein is;' implying that, even if there are gods, they are under Him, that is, under the God of the Jews. And again: 'The Lord your God, He is God in heaven above, and upon the earth beneath, and there is none other except Him.' And somewhere else the Scripture says to the Jewish multitude, 'The Lord your God is God of gods;' so that, even if there are gods, they are under the God of the Jews.
And somewhere else the Scripture says in regard to Him, God, the great and true, who regards not persons, nor takes reward, He executes the judgment of the fatherless and widow.' The Scripture, in calling the God of the Jews great and true, and executing judgment, marked out the others as small, and not true. But also somewhere else the Scripture says, 'As I live, says the Lord, there is no other God but me. I am the first, I am after this; except me there is no God.' And again: 'You shall fear the Lord your God, and Him only shall you serve.'
And again: 'Hear, O Israel, the Lord your God is one Lord.' And many passages besides seal with an oath that God is one, and except Him there is no God. Whence I wonder how, when so many passages testify that there is one God, you say that there are many.”
Source: Clementine Homilies (New Advent)