John Chrysostom — as excerpted in the Catena Aurea on Matthew 7:15-20

“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.”