Tertullian, Against Marcion 20

We also need to defend that very clear proof against the clamor of the opposing side. They claim Marcion didn't so much innovate the rule by separating Law and Gospel as revert to a rule previously adulterated. Oh Christ, most patient Lord, you endured the perversion of your preaching for so many years, until Marcion finally came to your rescue! They even pit Peter and the other pillars of the apostleship against Paul, because Paul rebuked them for not walking straight toward the truth of the Gospel. This is the same Paul who was still raw in grace and trembling lest he had run or would run in vain when he first conferred with the earlier apostles. So if, while still burning with zeal like a new convert, he judged something in their conduct reprehensible toward Judaism, then—once convinced—he would later become all things to all men to win them all: to Jews as a Jew, to those under the Law as under the Law. You want that rebuke of mere conduct—a rebuke that would please his future accuser—to be suspected as a falsification of his preaching toward God. But on the contrary, concerning the unity of preaching we read above, they joined right hands and agreed on a division of duties in the fellowship of the Gospel. As Paul says elsewhere: "Whether I or they, so we preach." Even when he describes certain false brothers who slipped in to divert the Galatians to a different gospel, he shows that this adulteration of the Gospel did not aim at transferring faith to another God and Christ, but at preserving the discipline of the Law—detecting those who insisted on circumcision and on observing times, days, months, and years of Jewish ceremonies. They should have known these were already excluded, according to the renewed disposition of the Creator, who had long before preached this very thing through his prophets. Through Isaiah: "The old things have passed away; behold, I am doing new things" (Isaiah 43:19). And through Jeremiah: "I will make a covenant, not like the one I made with your fathers when I led them out of Egypt" (Jeremiah 31:32). And elsewhere: "Renew a newness for yourselves, and be circumcised to your God, and circumcise the foreskins of your hearts." Therefore the apostle, now establishing this circumcision and this newness, was also dissuading those old ceremonies—ceremonies whose own Creator had once declared would cease. Through Hosea: "I will turn away all her delights, her feast days, new moons, sabbaths, and all her ceremonies" (Hosea 2:13). For so also through Isaiah: "I do not endure your new moons and sabbaths and great days; your festivals and fasts and feast days my soul hates." But if the Creator had already rejected all these things long ago, and the apostle now pronounced them rejected, the apostle's own verdict—consistent with the Creator's decrees—proves he did not preach another God than the one whose decrees he wanted recognized. He brands as false both the apostles and the brothers in this cause who would transfer the Gospel of Christ the Creator from the newness foretold by the Creator to the antiquity rejected by the Creator.