“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 therefore, 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 among them unguarded.” Then that you might 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 brings forth good fruit, but the corrupt tree brings forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.”
Now what He says 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 you should 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,” says 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 says 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 has deprived them of all excuse. “For you can not say, 'I am deceived and beguiled;' since I have given you 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.”
Source: Homilies on the Gospel of St. Matthew (New Advent)