20 In the first place let me ask our opponents if these ancient righteous men, Enoch and Seth, whom Faustus mentions particularly, and any others who lived before Moses, or even, if you choose, before Abraham, were angry with their brother without a cause, or said to their brother, Thou fool. If not, why may they not have taught these things as well as preached them? And if they taught these things, how can Christ be said to have fulfilled their righteousness or their teaching, any more than that of Moses, by adding, "But I say unto you, if any man is angry with his brother, or if he says Racha, or if he says, You fool, he shall be in danger of the judgment, or of the council, or of hell-fire," since these men did these very things themselves, and enjoined them upon others?
Will it be said that they were ignorant of its being the duty of a righteous man to restrain his passion, and not to provoke his brother with angry abuse; or that, knowing this, they were unable to act accordingly? In that case, they deserved the punishment of hell, and could not have been righteous. But no one will venture to say that in their righteousness there was such ignorance of duty, and such a want of self-control, as to make them liable to the punishment of hell. How, then, can Christ be said to have fulfilled the law, by which these men lived by means of adding things without which they could have had no righteousness at all?
Will it be said that a hasty temper and bad language are sinful only since the time of Christ, while formerly such qualities of the heart and speech were allowable; as we find some institutions vary according to the times, so that what is proper at one time is improper at another, and vice versa? You will not be so foolish as to make this assertion. But even were you to do so, the reply will be that, according to this idea, Christ came not to fulfill what was defective in the old law, but to institute a law which did not previously exist; if it is true that with the righteous men of old it was not a sin to say to their brother, You fool, which Christ pronounces so sinful, that whoever does so is in danger of hell.
So, then, you have not succeeded in finding any law of which it can be said that Christ supplied its defect by these additions.
Source: Reply to Faustus the Manichaean (New Advent)