7 If you have learned from Manichæus the willful impiety of admitting only those parts of the Gospel which do not contradict your errors, while you reject the rest, we have learned from the apostle the pious caution of looking on every one as accursed that preaches to us another gospel than that which we have received. Hence Catholic Christians look upon you as among the tares; for, in the Lord's exposition of the meaning of the tares, they are not falsehood mixed with truth in the Scriptures, but children of the wicked one— that is, people who imitate the deceitfulness of the devil.
It is not true that Catholic Christians believe everything; for they do not believe Manichæus or any of the heretics. Nor do they condemn the use of human reason; but what you call reasoning they prove to be fallacious. Nor do they think it profane to distinguish truth from falsehood; for they distinguish between the truth of the Catholic faith and the falsehood of your doctrines. Nor do they fear to separate good from evil; but they contend that evil, instead of being natural, is unnatural.
They know nothing of your race of darkness, which, you say, is produced from a principle of its own, and fights against the kingdom of God, and of which your god seems really to be more frightened than children are of ghosts; for, according to you, he covered himself with a veil, that he might not see his own members taken and plundered by the assault of the enemy. To conclude, Catholic Christians are in no difficulty regarding the words of Christ, though in one sense they may be said not to observe the law and the prophets; for by the grace of Christ they keep the law by their love to God and man; and on these two commandments hang all the law and the prophets. Besides, they see in Christ and the Church the fulfillment of all the prophecies of the Old Testament, whether in the form of actions, or of symbolic rites, or of figurative language.
So we neither join in superstitious follies, nor declare this verse false; nor deny that we are followers of Christ; for on those principles which I have set forth to the best of my power, the law and the prophets which Christ came not to destroy, but to fulfill, are no other than those recognized by the Church.
Source: Reply to Faustus the Manichaean (New Advent)