78 Here, therefore, those who promise a wisdom and a knowledge of the truth which they do not possess, are especially to be guarded against; as, for instance, heretics, who frequently commend themselves on account of their fewness. And hence, when He had said that there are few who find the strait gate and the narrow way, lest they [the heretics] should falsely substitute themselves under the pretext of their fewness, He immediately added, “Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.” But such parties do not deceive the single eye, which knows how to distinguish a tree by its fruits. For He says: “You shall know them by their fruits.” Then He adds the similitudes: “Do men gather grapes of thorns, or figs of thistles? Even so, every good tree brings forth good fruit; but a corrupt tree brings forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that brings not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits you shall know them.”
79. And in [the interpretation of] this passage we must be very much on our guard against the error of those who judge from these same two trees that there are two original natures, the one of which belongs to God, but the other neither belongs to God nor springs from Him. And this error has both been already discussed in other books [of ours] very copiously, and if that is still too little, will be discussed again; but at present we have merely to show that the two trees before us do not help them. In the first place, because it is so clear that He is speaking of men, that whoever reads what goes before and what follows will wonder at their blindness. Secondly, they fix their attention on what is said, “A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit,” and therefore think that neither can it happen that an evil soul should be changed into something better, nor a good one into something worse; as if it were said, A good tree cannot become evil, nor an evil tree good. But it is said, “A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.” For the tree is certainly the soul itself, i.e. the man himself, but the fruits are the works of the man; an evil man, therefore, cannot perform good works, nor a good man evil works. If an evil man, therefore, wishes to perform good works, let him first become good. So the Lord Himself says in another passage more plainly: “Either make the tree good, or make the tree bad.” But if He were figuratively representing the two natures of such parties by these two trees, He would not say, “Make:” for who of the sons of men can make a nature? Then also in that passage, when He had made mention of these two trees, He added, “You hypocrites, how can you, being evil, speak good things?” As long, therefore, as any one is evil, he cannot bring forth good fruits; for if he were to bring forth good fruits, he would no longer be evil. So it might most truly have been said, snow cannot be warm; for when it begins to be warm, we no longer call it snow, but water. It may therefore come about, that what was snow is no longer so; but it cannot happen that snow should be warm. So it may come about, that he who was evil is no longer evil; it cannot, however, happen that an evil man should do good. And although he is sometimes useful, this is not the man's own doing; but it is done through him, in virtue of the arrangements of divine providence: as, for instance, it is said of the Pharisees, “What they bid you, do; but what they do, do not consent to do.” This very circumstance, that they spoke things that were good, and that the things which they spoke were usefully listened to and done, was not a matter belonging to them: for, says He, “they sit in Moses' seat.” It was, therefore, when engaged through divine providence in preaching the law of God, that they were able to be useful to their hearers, although they were not so to themselves. Respecting such it is said in another place by the prophet, “They have sown wheat, but shall reap thorns;” because they teach what is good, and do what is evil. Those, therefore, who listened to them, and did what was said by them, did not gather grapes of thorns, but through the thorns gathered grapes of the vine: just as, were any one to thrust his hand through a hedge, or were at least to gather a grape from a vine which was entangled in a hedge, that would not be the fruit of the thorns, but of the vine.
80. The question, indeed, is most rightly put, What are the fruits He would wish us to attend to, whereby we might know the tree? For many reckon among the fruits certain things which belong to the sheep's clothing, and in this way are deceived by wolves: as, for instance, either fastings, or prayers, or almsgivings; but unless all of these things could be done even by hypocrites, He would not say above, “Take heed that you do not your righteousness before men, to be seen of them.” And after prefixing this sentence, He goes on to speak of those very three things, almsgiving, prayer, fasting. For many give largely to the poor, not from compassion, but from vanity; and many pray, or rather seem to pray, while not keeping God in view, but desiring to please men; and many fast, and make a wonderful show of abstinence before those to whom such things appear difficult, and by whom they are reckoned worthy of honour: and catch them with artifices of this sort, while they hold up to view one thing for the purpose of deceiving, and put forth another for the purpose of preying upon or killing those who cannot see the wolves under that sheep's clothing. These, therefore, are not the fruits by which He admonishes us that the tree is known. For such things, when they are done with a good intention in sincerity, are the appropriate clothing of sheep; but when they are done in wicked deception, they cover nothing else but wolves. But the sheep ought not on this account to hate their own clothing, because the wolves often conceal themselves therein.
81. What the fruits are by the finding of which we may know an evil tree, the apostle tells us: “Now the works of the flesh are manifest, which are these; adulteries, fornications, uncleanness, lasciviousness, idolatry, witchcraft, hatreds, variances, emulations, wrath, strife, seditions, heresies, envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.” And what the fruits are by which we may know a good tree, the very same apostle goes on to tell us: “But the fruit of the Spirit is love, joy, peace, long-suffering, gentleness, goodness, faith, meekness, temperance.” It must be known, indeed, that “joy” stands here in a strict and proper sense; for bad men are, strictly speaking, not said to rejoice, but to make extravagant demonstrations of joy: just as we have said above, that “will” which the wicked do not possess, stands in a strict sense where it is said, “All things whatsoever ye would that men should do to you, do ye even so to them.” In accordance with that strict sense of the word, in virtue of which joy is spoken of only in the good, the prophet also speaks, saying: “Rejoicing is not for the wicked, says the Lord.” So also “faith” stands, not certainly as meaning any kind of it, but true faith: and the other things which find a place here have certain resemblances of their own in bad men and deceivers; so that they entirely mislead, unless one has the pure and single eye by which he may know such things. It is accordingly the best arrangement, that the cleansing of the eye is first discussed, and then mention is made of what things were to be guarded against.
Source: Our Lord's Sermon on the Mount (New Advent)