Matthew 7:18

(Verse 18 and following) A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Let us ask the heretics who claim that there are two contradictory natures, if according to their understanding, a good tree can never bear bad fruit, then how did Moses, a good tree, sin by striking the rock (Deut. 32)? And how did David, after killing Uriah, sleep with Bathsheba (2 Sam. 11)? And how did Peter, during the Passion of the Lord, deny Him, saying: 'I do not know the man' (Matt. 26:72)? Or what connection is there between Jethro, the father-in-law of Moses, a bad tree who did not believe in the God of Israel, giving good advice to Moses (Exod. 18)? And how did Achior, in speaking to Holofernes, say something useful (Judith 5)? And as the comedian said (which the Apostle confirmed): 'Evil company corrupts good morals' (1 Cor. 5). And when they do not find what to answer, let us infer that Judas (John 13) , once a good tree, bore bad fruits after betraying (or handing over) the Savior; and that Saul (Philippians 3) , a bad tree at the time he persecuted the Church of Christ, bore good fruits afterwards, when he was transformed from a persecutor into a vessel of election (Acts 9) . Therefore, a good tree does not bear bad fruits as long as it perseveres in the pursuit of goodness; and a bad tree remains in the fruits of sin as long as it does not convert to repentance. For no one remains in what he was, he begins to be what he has not yet begun.

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.

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. […]

(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. […]

Here the question will arise How creatures were made good out of it, which were formed without any change at all? How occurs the seed of what is good, nay, very good, in that which is evil, nay, very evil? Surely a good tree does not produce evil fruit, since there is no God who is not good; nor does an evil tree yield good fruit, since there is not Matter except what is very evil.