Mt 7:15-20
"Beware of false prophets, who come to you in sheep's clothing, but underneath are ravenous wolves. By their fruits you will know them. Do people pick grapes from thornbushes, or figs from thistles? Just so, every good tree bears good fruit, and a rotten tree bears bad fruit. A good tree cannot bear bad fruit, nor can a rotten tree bear good fruit. Every tree that does not bear good fruit will be cut down and thrown into the fire. So by their fruits you will know them.
