The Five Books Against Marcion by Tertullian
Chapter XXVI.--The Oath of God: Its Meaning. Moses, When Deprecating God's Wrath Against Israel, a Type of Christ.
But God also swears. Well, is it, I wonder, by the God of Marcion? No, no, he says; a much vainer oath -- by Himself! What was He to do, when He knew of no other God; especially when He was swearing to this very point, that besides himself there was absolutely no God? Is it then of swearing falsely that you convict Him, or of swearing a vain oath? But it is not possible for him to appear to have sworn falsely, when he was ignorant, as you say he was, that there was another God. For when he swore by that which he knew, he really committed no perjury. But it was not a vain oath for him to swear that there was no other God. It would indeed be a vain oath, if there had been no persons who believed that there were other Gods, like the worshippers of idols then, and the heretics of the present day. Therefore He swears by Himself, in order that you may believe God, even when He swears that there is besides Himself no other God at all. But you have yourself, O Marcion, compelled God to do this. For even so early as then were you foreseen. Hence, if He swears both in His promises and His threatenings, and thus extorts faith which at first was difficult, nothing is unworthy of God which causes men to believe in God. But (you say) God was even then mean enough in His very fierceness, when, in His wrath against the people for their consecration of the calf, He makes this request of His servant Moses: |Let me alone, that my wrath may wax hot against them, and that I may consume them; and I will make of thee a great nation.| Accordingly, you maintain that Moses is better than his God, as the deprecator, nay the averter, of His anger. |For,| said he, |Thou shalt not do this; or else destroy me along with them.| Pitiable are ye also, as well as the people, since you know not Christ, prefigured in the person of Moses as the deprecator of the Father, and the offerer of His own life for the salvation of the people. It is enough, however, that the nation was at the instant really given to Moses. That which he, as a servant, was able to ask of the Lord, the Lord required of Himself. For this purpose did He say to His servant, |Let me alone, that I may consume them,| in order that by his entreaty, and by offering himself, he might hinder (the threatened judgment), and that you might by such an instance learn how much privilege is vouchsafed with God to a faithful man and a prophet.