But the Christian verity has distinctly declared this principle, “God is not, if He is not one;” because we more properly believe that that has no existence which is not as it ought to be. In order, however, that you may know that God is one, ask what God is, and you will find Him to be not otherwise than one. So far as a human being can form a definition of God, I adduce one which the conscience of all men will also acknowledge,—that God is the great Supreme existing in eternity, unbegotten, unmade without beginning, without end.
…Whatever other god, then, you may introduce, you will at least be unable to maintain his divinity under any other guise, than by ascribing to him too the property of Godhead—both eternity and supremacy over all. How, therefore, can two great Supremes co-exist, when this is the attribute of the Supreme Being, to have no equal,—an attribute which belongs to One alone, and can by no means exist in two?
Tertullian
Five Books Against Marcion, Book I
Chapter III