5 Since you hold to the divine authorship of the Old as well as of the New Testament, it would surely be more consistent and more becoming, as you do not obey its precepts, to confess that it has been corrupted by improper additions, than to treat it so contemptuously, if it is genuine and uncorrupted. Accordingly, my explanation of your neglect of the requirements of the Old Testament has always been, and still is, that you are either wise enough to reject them as spurious, or that you have the boldness and irreverence to disregard them if they are true.
At any rate, when you would oblige me to believe everything contained in the documents of the New Testament because I receive the Testament itself, you should consider that, though you profess to receive the Old Testament, you in your heart disbelieve many things in it. Thus, you do not admit as true or authoritative the declaration of the Old Testament, that every one that hangs on a tree is accursed, for this would apply to Jesus; or that every man is accursed who does not raise up seed in Israel, for that would include all of both sexes devoted to God; or that whoever is not circumcised in the flesh of his foreskin will be cut off from among his people, for that would apply to all Christians; or that whoever breaks the Sabbath must be stoned to death; or that no mercy should be shown to the man who breaks a single precept of the Old Testament.
If you really believe these things as certainly enjoined by God, you would, in the time of Christ, have been the first to assail Him, and you would now have no quarrel with the Jews, who, in persecuting Christ with heart and soul, acted in obedience to their own God.
Source: Reply to Faustus the Manichaean (New Advent)