John Chrysostom
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.
But as speaking not for Himself but for the Holy Spirit, He accordingly rebukes them, saying, Generation of vipers, how can ye being evil speak good things? This is both a rebuke of them, and a proof in their own characters of those things which had been said. As though He had said, So ye being corrupt trees cannot bring forth good fruit. I do not wonder then that you thus speak, for you are ill nourished of ill parentage, and have an evil mind. And observe He said not, How can ye speak good things, seeing ye are a generation of vipers? for these two are not connected together; but He said, How can ye being evil speak good things? He calls them generation of vipers, because they made boast of their forefathers; in order therefore to cut off this their pride, He shuts them out of the race of Abraham, assigning them a parentage corresponding to their characters.