The Five Books Against Marcion by Tertullian
Chapter III.--The Unity of God. He is the Supreme Being, and There Cannot Be a Second Supreme.
The principal, and indeed the whole, contention lies in the point of number: whether two Gods may be admitted, by poetic licence (if they must be), or pictorial fancy, or by the third process, as we must now add, of heretical pravity. 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. For such a condition as this must needs be ascribed to that eternity which makes God to be the great Supreme, because for such a purpose as this is this very attribute in God; and so on as to the other qualities: so that God is the great Supreme in form and in reason, and in might and in power. Now, since all are agreed on this point (because nobody will deny that God is in some sense the great Supreme, except the man who shall be able to pronounce the opposite opinion, that God is but some inferior being, in order that he may deny God by robbing Him of an attribute of God), what must be the condition of the great Supreme Himself? Surely it must be that nothing is equal to Him, i.e. that there is no other great supreme; because, if there were, He would have an equal; and if He had an equal, He would be no longer the great Supreme, now that the condition and (so to say) our law, which permits nothing to be equal to the great Supreme, is subverted. That Being, then, which is the great Supreme, must needs be unique, by having no equal, and so not ceasing to be the great Supreme. Therefore He will not otherwise exist than by the condition whereby He has His being; that is, by His absolute uniqueness. Since, then, God is the great Supreme, our Christian verity has rightly declared, |God is not, if He is not one.| Not as if we doubted His being God, by saying, He is not, if He is not one; but because we define Him, in whose being we thoroughly believe, to be that without which He is not God; that is to say, the great Supreme. But then the great Supreme must needs be unique. This Unique Being, therefore, will be God -- not otherwise God than as the great Supreme; and not otherwise the great Supreme than as having no equal; and not otherwise having no equal than as being Unique. 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?