John Chrysostom

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. …

Homily on the Gospel of Matthew 23 ↗

Theophylact of Ohrid

"Do men gather grapes from thorns, or figs from thistles? Even so, every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit." The hypocrites are called thorns and thistles: they are like thorns in that they prick unexpectedly; they are like thistles in their cunning and deviousness. The corrupt tree is anyone who has been corrupted by a pleasure-loving and dissolute life.

Commentary on Matthew ↗

Augustine of Hippo

Wherefore it is justly asked, what fruits then He would have us look to? For many esteem among fruits some things which pertain to the sheep's clothing, and in this manner are deceived concerning wolves. For they practise fasting, almsgiving, or praying, which they display before men, seeking to please those to whom these things seem difficult. These then are not the fruits by which He teaches us to discern them. …

Catena Aurea by Aquinas ↗

Thomas Aquinas

You will know them by their fruits, i.e., actions. But it seems to be the opposite, because they have sheep's clothing, and clothing is works. Therefore, they will be known from them. Chrysostom: "Fruit is the confession of faith." Hence, if they confess the faith, they are not heretics: "For the fruit of the light is in all goodness and justice and truth" (Eph 5:9). But if it is taken for pretenders, then it is explained so that by clothing is meant external works: "But the fruit of the spirit is charity, joy and peace" (Gal 5:22). …

Commentary on Matthew ↗