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.
But that none should say, An evil tree brings forth indeed evil fruit, but it brings forth also good, and so it becomes hard to discern, as it has a two-fold produce; on this account He adds, A good tree cannot bring forth evil fruit, neither can an evil tree bring forth good fruit.