Charles Ellicott Commentary Matthew 7:17-18

"Even so every good tree bringeth forth good fruit; but the corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit." — Matthew 7:17-18 (ASV)

Even so every good tree... —The two verses state nearly the same fact, but each presents a different aspect. First, the fact is stated as a matter of practical experience; then, it is referred to as a necessary law. If the tree is corrupt—that is, rotten or decayed at the core—it cannot bring forth good fruit.

If there is falseness in the teaching or in the man, it will sooner or later show itself in his life. At that point, even if we judge the doctrine on other grounds, we should lose confidence in the teacher's guidance.