Celsus adds, that it was foretold to the Jews, that if they did not obey the law, they would be treated in the same way as they treated their enemies; and then he quotes from the teaching of Christ some precepts which he considers contrary to those of the law, and uses that as an argument against us. But before proceeding to this point, we must speak of that which precedes. We hold, then, that the law has a twofold sense,— the one literal, the other spiritual,— as has been shown by some before us.
Of the first or literal sense it is said, not by us, but by God, speaking in one of the prophets, that “the statutes are not good, and the judgments not good;” whereas, taken in a spiritual sense, the same prophet makes God say that “His statutes are good, and His judgments good.” Yet evidently the prophet is not saying things which are contradictory of each other. Paul in like manner says, that “the letter kills, and the spirit gives life,” meaning by “the letter” the literal sense, and by “the spirit” the spiritual sense of Scripture.
We may therefore find in Paul, as well as in the prophet, apparent contradictions. Indeed, if Ezekiel says in one place, “I gave them commandments which were not good, and judgments whereby they should not live,” and in another, “I gave them good commandments and judgments, which if a man shall do, he shall live by them,” Paul in like manner, when he wishes to disparage the law taken literally, says, “If the ministration of death, written and engraven in stones, was glorious, so that the children of Israel could not steadfastly behold the face of Moses for the glory of his countenance, which glory was to be done away; how shall not the ministration of the Spirit be rather glorious?” But when in another place he wishes to praise and recommend the law, he calls it “spiritual,” and says, “We know that the law is spiritual;” and, “Wherefore the law is holy, and the commandment holy, and just, and good.”
Source: Against Celsus (New Advent)