5 But we reject those false teachers whose Christ is false, or rather, whose Christ never existed. For we have a Christ true and truthful, foretold by the prophets, preached by the apostles, who in innumerable places refer to the testimonies of the law and the prophets in support of their preaching. Paul, in one short sentence, gives the right view of this subject. "Now," he says, "the righteousness of God without the law is manifested, being witnessed by the law and the prophets." What prophets, if not of Israel, to whom, as he expressly says, pertain the covenants, and the giving of the law, and the promises?
And what promises, but about Christ? Elsewhere, speaking of Christ, he says concisely: "All the promises of God are in Him yea." Paul tells me that the giving of the law pertained to the Israelites. He also tells me that Christ is the end of the law for righteousness to every one that believes. He also tells me that all the promises of God are in Christ yea. And you tell me that the prophets of Israel foretold nothing of Christ. Shall I believe the absurdities of Manichæus relating a vain and long fable in opposition to Paul? Or shall I believe Paul when he forewarns us: "If any man preach to you another gospel than that which we have preached, let him be accursed?"
Source: Reply to Faustus the Manichaean (New Advent)