But some one may contend that two great Supremes may exist, distinct and separate in their own departments; and may even adduce, as an example, the kingdoms of the world, which, though they are so many in number, are yet supreme in their several regions. Such a man will suppose that human circumstances are always comparable with divine ones. Now, if this mode of reasoning be at all tolerable, what is to prevent our introducing, I will not say a third god or a fourth, but as many as there are kings of the earth? Now it is God that is in question, whose main property it is to admit of no comparison with Himself. Nature itself, therefore, if not an Isaiah, or rather God speaking by Isaiah, will deprecatingly ask, “To whom will ye liken me?” Human circumstances may perhaps be compared with divine ones, but they may not be with God. God is one thing, and what belongs to God is another thing. …
The inevitable conclusion at which we arrive, then, on this point is this: either we must deny that God is the great Supreme, which no wise man will allow himself to do; or say that God has no one else with whom to share His power.
The Five Books Against Marcion