Marcion's Object in Adulterating the Gospel. No Difference Between the Christ of the Creator and the Christ of the Gospel. No Rival Christ Admissible. The Connection of the True Christ with the Dispensation of the Old Testament Asserted
But we now advance a step further on, and challenge (as we promised to do) the very Gospel of Marcion, with the intention of thus proving that it has been adulterated. For it is certain that the whole aim at which he has strenuously laboured even in the drawing up of his Antitheses, centres in this, that he may establish a diversity between the Old and the New Testaments, so that his own Christ may be separate from the Creator, as belonging to this rival god, and as alien from the law and the prophets.
It is certain, also, that with this view he has erased everything that was contrary to his own opinion and made for the Creator, as if it had been interpolated by His advocates, while everything which agreed with his own opinion he has retained. The latter statements we shall strictly examine; and if they shall turn out rather for our side, and shatter the assumption of Marcion, we shall embrace them. It will then become evident, that in retaining them he has shown no less of the defect of blindness, which characterizes heresy, than he displayed when he erased all the former class of subjects.
Such, then, is to be the drift and form of my little treatise; subject, of course, to whatever condition may have become requisite on both sides of the question. Marcion has laid down the position, that Christ who in the days of Tiberius was, by a previously unknown god, revealed for the salvation of all nations, is a different being from Him who was ordained by God the Creator for the restoration of the Jewish state, and who is yet to come. Between these he interposes the separation of a great and absolute difference— as great as lies between what is just and what is good; as great as lies between the law and the gospel; as great, (in short,) as is the difference between Judaism and Christianity.
Hence will arise also our rule, by which we determine that there ought to be nothing in common between the Christ of the rival god and the Creator; but that (Christ) must be pronounced to belong to the Creator, if He has administered His dispensations, fulfilled His prophecies, promoted His laws, given reality to His promises, revived His mighty power, remoulded His determinations, expressed His attributes, His properties. This law and this rule I earnestly request the reader to have ever in his mind, and so let him begin to investigate whether Christ be Marcion's or the Creator's.
Source: Against Marcion (New Advent)