John Chrysostom
Catena Aurea · Matthäusevangelium 7
Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves. Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits ye shall know them.
Yet He may seem here to have aimed under the title of false prophets, not so much at the heretic, as at those who, while their life is corrupt, yet wear an outward face of virtuousness; whence it is said, By their fruits ye shall know them. For among heretics it is possible many times to find a good life, but among those I have named never.