John Chrysostom

"For by their fruits," saith He, "ye shall know them." And what commends our own life? Is it indeed a display of miracles, or the perfection of an excellent conversation? Very evidently it is the second; but as to the miracles, they both have their origin from hence, and terminate herein. For both He that shows forth an excellent life, draws to Himself this gift, and he that receives the gift, receives it for this end, that he may amend other men's lives. …

Homily on the Gospel of Matthew 46 ↗

Theophylact of Ohrid

"Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits ye shall know them." He addresses the Jews, speaking the same words as did John (Mt. 3:10). Jesus likens man to a tree. For by the introduction of a graft, a fruitless tree can bear fruit; so, too, a sinful and fruitless man when engrafted with Christ can bear fruits of virtue.

Commentary on Matthew ↗

Augustine of Hippo

On this point, one must carefully avoid the error of those who think that they find in these two trees a reason for believing that there are two natures and that one of them belongs to the nature of God but that the other neither belongs to God nor depends on him. This error has been rather fully discussed in other books, and if that is not sufficient it will receive still further treatment later. But we must now show that these two trees furnish no argument in support of it. …

Sermon On The Mount 2.24.79 ↗

Thomas Aquinas

But what will happen to the bad tree? Every tree that does not bear good fruit is cut down, because if it does not bear or if it omits doing what it can, it will be cut down: "If anyone does not abide in me, he will be cast out, and they will gather him up and throw him into the fire, and he will burn" (Jn 15:6). Hence in Luke (8:7) it is said of the fig tree which the Lord commanded to be cut down and taken away: "Take away the wicked one, lest it see God's glory."

Commentary on Matthew ↗