Matthew 7:19

Others maintain that these things are said with reference to teachers of foreign doctrines and to people who mingle lies with the truth. Just as one cannot gather evil fruit from a tree that produces good fruit, neither can one gather grapes or figs from one that bears thorns; similarly, from an evil mindset, one does not hear good words, nor would you expect evil teaching to come from a pious teacher. But in protecting the others so that they may not be easily deluded, Jesus at the same time deters those who would attempt to teach evil things. For, he says, "every tree that does not bear good fruit is cut down and cast into the fire." Just as fruitless trees are thrown into the fire, so human beings who do not bear the fruits of religion with their mouths will consequently receive punishment.

"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. Since even Christ for this end wrought those miracles, that having made Himself thereby credible, and drawn men unto Him, He might bring virtue into our life. Wherefore also He lays more stress of the two on this. For He is not at all satisfied with the signs only, but He also threatens hell, and promises a kingdom, and lays down those startling laws, and all things He orders to this end, that He may make us equal to the angels.

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. First of all, in this similitude the Lord is speaking about two kinds of persons. This is so obvious that if anyone will but read the passages in the context of those that precede and follow it, he will be amazed at the blindness of those who would misinterpret it impersonally. Again, they fix their attention on the saying, "A good tree cannot bear bad fruit, nor can a bad tree bear good fruit," and then they think that an evil soul cannot be changed into a better or a good soul into a worse. As though, in truth, the saying were "A good tree cannot become a bad tree, nor a bad tree become a good tree!" But what has been said is that "a good tree cannot bear bad fruit, and a bad tree cannot bear good fruit." The tree, of course, is the soul itself—that is, the person—and the fruits are the person's works. So a bad person cannot perform good works, nor can a good person perform bad works.