Proof that God is one and not many
We have, then, adequately demonstrated that there is a God, and that His essence is incomprehensible. But that God is one and not many is no matter of doubt to those who believe in the Holy Scriptures. For the Lord says in the beginning of the Law: I am the Lord your God, which have brought you out of the land of Egypt. You shall have no other Gods before Me. And again He says, Hear, O Israel, the Lord our God is one Lord. And in Isaiah the prophet we read, For I am the first God and I am the last, and beside Me there is no God. Before Me there was not any God, nor after Me will there be any God, and beside Me there is no God. And the Lord, too, in the holy gospels speaks these words to His Father, And this is life eternal, that they may know You the only true God. But with those that do not believe in the Holy Scriptures we will reason thus.
The Deity is perfect, and without blemish in goodness, and wisdom, and power, without beginning, without end, everlasting, uncircumscribed, and in short, perfect in all things. Should we say, then, that there are many Gods, we must recognise difference among the many. For if there is no difference among them, they are one rather than many. But if there is difference among them, what becomes of the perfectness? For that which comes short of perfection, whether it be in goodness, or power, or wisdom, or time, or place, could not be God. But it is this very identity in all respects that shows that the Deity is one and not many.
Again, if there are many Gods, how can one maintain that God is uncircumscribed? For where the one would be, the other could not be.
Further, how could the world be governed by many and saved from dissolution and destruction, while strife is seen to rage between the rulers? For difference introduces strife. And if any one should say that each rules over a part, what of that which established this order and gave to each his particular realm? For this would the rather be God. Therefore, God is one, perfect, uncircumscribed, maker of the universe, and its preserver and governor, exceeding and preceding all perfection.
Moreover, it is a natural necessity that duality should originate in unity.
Source: Exposition of the Faith (New Advent)