Gregory the Great
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.
(Mor. xxxi. 14.) Also the hypocrite is restrained by peaceful times of Holy Church, and therefore appears clothed with godliness; but let any trial of faith ensue, straight the wolf ravenous at heart strips himself of his sheep’s skin, and shews by persecuting how great his rage against the good.