Even so every good tree.... — The two verses state nearly the same fact, but each presents a different aspect. First it is stated as a matter of practical experience, then the general fact is referred to a necessary law. If the tree is corrupt, i.e., 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, and then, even though we judge of the doctrine on other ground, we should cease to feel confidence in the guidance of the teacher.
Related Commentaries of Matthew 7:17
Matthew 7:17-18
17 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.