Matthew 7:18

But that none should say, An evil tree brings forth indeed evil fruit, but it brings forth also good, and so it becomes hard to discern, as it has a two-fold produce; on this account He adds, A good tree cannot bring forth evil fruit, neither can an evil tree bring forth good fruit.

We would ask those heretics who affirm that there are two natures directly opposed to each other, if they admit that a good tree cannot bring forth evil fruit, how it was possible for Moses, a good tree, to sin as he did at the water of contradiction? Or for Peter to deny his Lord in the Passion, saying, I know not the man? Or how, on the other hand, could Moses' father-in-law, an evil tree, inasmuch as he believed not in the God of Israel, give good counsel?

And by the figure of "false prophets," here, I think He shadows out not the heretics, but them that are of a corrupt life, yet wear a mask of virtue; whom the generality are wont to call by the name of impostors. Wherefore He also said further, "By their fruits ye shall know them." For amongst heretics one may often find actual goodness, but amongst those whom I was mentioning, by no means. "What then," it may be said, "if in these things too they counterfeit?" "Nay, they will be easily detected; for such is the nature of this way, in which I commanded men to walk, painful and irksome; but the hypocrite would not choose to take pains, but to make a show only; wherefore also he is easily convicted." Thus, inasmuch as He had said, "there be few that find it," He clears them out again from among those, who find it not, yet feign so to do, by commanding us not to look to them that wear the masks only, but to them who in reality pursue it. "But wherefore," one may say, "did He not make them manifest, but set us on the search for them?" That we might watch, and be ever prepared for conflict, guarding against our disguised as well as against our open enemies: which kind indeed Paul also was intimating, when he said, that "by their good words they deceive the hearts of the simple." Let us not be troubled therefor, when we see many such even now. Nay, for this too Christ foretold from the beginning. And see His gentleness: how He said not, "Punish them," but, "Be not hurt by them," "Do not fall amongst them unguarded." Then that thou mightest not say, "it is impossible to distinguish that sort of men," again He states an argument from a human example, thus saying, "Do men gather grapes of thorns, or figs of thistles? even so every good tree bringeth forth good fruit, but the corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit." Now what He saith is like this: they have nothing gentle nor sweet; it is the sheep only so far as the skin; wherefore also it is easy to discern them. And lest thou shouldest have any the least doubt, He compares it to certain natural necessities, in matters which admit of no result but one. In which sense Paul also said, "The carnal mind is death; for it is not subject to the law of God, neither indeed can be." And if He states the same thing twice, it is not tautology. But, lest any one should say, "Though the evil tree bear evil fruit, it bears also good, and makes the distinction difficult, the crop being twofold:" "This is not so," saith He, "for it bears evil fruit only, and never can bear good: as indeed in the contrary case also." "What then? Is there no such thing as a good man becoming wicked? And the contrary again takes place, and life abounds with many such examples." But Christ saith not this, that for the wicked there is no way to change, or that the good cannot fall away, but that so long as he is living in wickedness, he will not be able to bear good fruit. For he may indeed change to virtue, being evil; but while continuing in wickedness, he will not bear good fruit. What then? did not David, being good, bear evil fruit? Not continuing good, but being changed; since, undoubtedly, had he remained always what he was, he would not have brought forth such fruit. For not surely while abiding in the habit of virtue, did he commit what he committed. Now by these words He was also stopping the mouths of those who speak evil at random, and putting a bridle on the lips of all calumniators. I mean, whereas many suspect the good by reason of the bad, He by this saying hath deprived them of all excuse. "For thou canst not say, 'I am deceived and beguiled;' since I have given thee exactly this way of distinguishing them by their works, having added the injunction to go to their actions, and not to confound all at random."

(Serm. in Mont. ii. 25.) In this place we must guard against the error of such as imagine that the two trees refer to two different natures; the one of God, the other not. But we affirm that they derive no countenance from these two trees; (Manichees, vid. infr.) as it will be evident to any who will read the context that He is speaking here of men. (De Civ. Dei, 12. 4.) These men of whom we have spoken are offended with these two natures, not considering them according to their true usefulness; whereas it is not by our advantage or disadvantage, but in itself considered, that nature gives glory to her Framer. All natures then that are, because they are, have their own manner, their own appearance, and as it were their own harmony, and are altogether good. (Serm. in Mont. ii. 25.) From this speech the Manichees suppose that neither can a soul that is evil be possibly changed for better, nor one that is good into worse. As though it had been, A good tree cannot become bad, nor a bad tree become good; whereas it is thus said, A good tree cannot bring forth evil fruit, nor the reverse. The tree is the soul, that is, the man himself; the fruit is the man's works. An evil man therefore cannot work good works, nor a good man evil works. Therefore if an evil man would work good things, let him first become good. But as long as he continues evil, he cannot bring forth good fruits. Like as it is indeed possible that what was once snow, should cease to be so; but it cannot be that snow should be warm; so it is possible that he who has been evil should be so no longer; but it is impossible that an evil man should do good. For though he may sometimes be useful, it is not he that does it, but it comes of Divine Providence superintending. (vid. Op. Imp. in. Jul. v. 40. &c. et alibi.) But as it is manifest that all evil works proceed from an evil will, as its fruits from an evil tree; so of this evil will itself whence will you say that it has sprung, except that the evil will of an angel sprung from an angel, of man from man? And what were these two before those evils arose in them, but the good work of God, a good and praiseworthy nature. See then out of good arises evil; nor was there any thing at all out of which it might arise but what was good. I mean the evil will itself, since there was no evil before it, no evil works, which could not come but from evil will as fruit from an evil tree. Nor can it be said that it sprung out of good in this way, because it was made good by a good God; for it was made of nothing, and not of God.

And man is denominated a good tree, or a bad, after his will, as it is good or bad. His fruit is his works, which can neither be good when the will is evil, nor evil when it is good.