32 Here we have a complete exposure of the falsehood of the Manichæans: for it is plain that the Lord did not in this matter teach one thing to the multitude, and another in private to His disciples. Here is abundant evidence that the error and deceit are in the Manichæans, and not in Moses, nor in Christ, nor in the doctrine taught figuratively in one Testament and plainly in the other—prophesied in one, and fulfilled in the other. How can the Manichæans say that the Catholics regard none of the things that Moses wrote, when in fact they observe them all, not now in the figures, but in what the figures were intended to foretell?
No one would say that one who reads the Scripture subsequently to its being written does not observe it because he does not form the letters which he reads. The letters are the figures of the sounds which he utters; and though he does not form the letters, he cannot read without examining them. The reason why the Jews did not believe in Christ, was because they did not observe even the plain literal precepts of Moses. So Christ says to them: "You pay tithe of mint and cummin, and omit the weightier matters of the law, mercy and judgment.
You strain out a gnat and swallow a camel. These ought ye to have done, and not to leave the other undone." So also He told them that by their traditions they made of none effect the commandment of God to give honor to parents. On account of this pride and perversity in neglecting what they understood, they were justly blinded, so that they could not understand the other things.
Source: Reply to Faustus the Manichaean (New Advent)