Jerome
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.
What is here spoken of false prophets we may apply to all whose dress and speech promise one thing, and their actions exhibit another. But it is specially to be understood of heretics, who by observing temperance, chastity, and fasting, surround themselves as it were with a garment of sanctity, but inasmuch as their hearts within them are poisoned, they deceive the souls of the more simple brethren.