Augustine of Hippo
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.
(Serm. in Mont. ii. 25.) In this place we must guard against the error of such as imagine that the two trees refer to two different natures; the one of God, the other not. But we affirm that they derive no countenance from these two trees; (Manichees, vid. infr.) as it will be evident to any who will read the context that He is speaking here of men.