The Five Books Against Marcion by Tertullian
Chapter VI.--Marcion Untrue to His Theory He Pretends that His Gods are Equal, But He Really Makes Them Diverse. Then, Allowing Their Divinity, Denies This Diversity.
Thus far our discussion seems to imply that Marcion makes his two gods equal. For while we have been maintaining that God ought to be believed as the one only great Supreme Being, excluding from Him every possibility of equality, we have treated of these topics on the assumption of two equal Gods; but nevertheless, by teaching that no equals can exist according to the law of the Supreme Being, we have sufficiently affirmed the impossibility that two equals should exist. For the rest, however, we know full well that Marcion makes his gods unequal: one judicial, harsh, mighty in war; the other mild, placid, and simply good and excellent. Let us with similar care consider also this aspect of the question, whether diversity (in the Godhead) can at any rate contain two, since equality therein failed to do so. Here again the same rule about the great Supreme will protect us, inasmuch as it settles the entire condition of the Godhead. Now, challenging, and in a certain sense arresting the meaning of our adversary, who does not deny that the Creator is God, I most fairly object against him that he has no room for any diversity in his gods, because, having once confessed that they are on a par, he cannot now pronounce them different; not indeed that human beings may not be very different under the same designation, but because the Divine Being can be neither said nor believed to be God, except as the great Supreme. Since, therefore, he is obliged to acknowledge that the God whom he does not deny is the great Supreme, it is inadmissible that he should predicate of the Supreme Being such a diminution as should subject Him to another Supreme Being. For He ceases (to be Supreme), if He becomes subject to any. Besides, it is not the characteristic of God to cease from any attribute of His divinity -- say, from His supremacy. For at this rate the supremacy would be endangered even in Marcion's more powerful god, if it were capable of depreciation in the Creator. When, therefore, two gods are pronounced to be two great Supremes, it must needs follow that neither of them is greater or less than the other, neither of them loftier or lowlier than the other. If you deny him to be God whom you call inferior, you deny the supremacy of this inferior being. But when you confessed both gods to be divine, you confessed them both to be supreme. Nothing will you be able to take away from either of them; nothing will you be able to add. By allowing their divinity, you have denied their diversity.