Augustine of Hippo
Catena Aurea · Matthäusevangelium 12
Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit. O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh. A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.
(Serm. 72. 1.) Or this is an admonition to ourselves that we should be good trees that we may be able to bring forth good fruit; Make the tree good, and its fruit good, is a precept of health to which obedience is necessary. But what He says, Make the tree corrupt, and its fruit corrupt, is not a command todo, but a warning to take heed, spoken against those who being evil thought that they could speak good things, or have good works; this the Lord declares is impossible. The man must be changed first, that his works may be changed; for if the man remains in that wherein he is evil, he cannot have good works; if he remains in that wherein he is good, he cannot have evil works. Christ found us all corrupt trees, but gave power to become sons of God to them that believe on His name.