A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.

People's New Testament

7:18 A good tree cannot bring forth bad fruit. The Lord points to the uniform law of nature. Every tree bears after its kind. The same principle holds good in the moral world. A good man will show forth good deeds, while a bad man will bear fruit according to his nature.

Wesley's Notes

7:18 A good tree cannot bring forth evil fruit, neither a corrupt tree good fruit - But it is certain, the goodness or badness here mentioned respects the doctrine, rather than the personal character. For a bad man preaching the good doctrine here delivered, is sometimes an instrument of converting sinners to God. Yet I do not aver, that all are true prophets who speak the truth, and thereby convert sinners. I only affirm, that none are such who do not.

Jamieson-Fausset-Brown Bible Commentary

18. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit-Obvious as is the truth here expressed in different forms-that the heart determines and is the only proper interpreter of the actions of our life-no one who knows how the Church of Rome makes a merit of actions, quite apart from the motives that prompt them, and how the same tendency manifests itself from time to time even among Protestant Christians, can think it too obvious to be insisted on by the teachers of divine truth. Here follows a wholesome digression.

Matthew Henry's Concise Commentary

7:15-20 Nothing so much prevents men from entering the strait gate, and becoming true followers of Christ, as the carnal, soothing, flattering doctrines of those who oppose the truth. They may be known by the drift and effects of their doctrines. Some part of their temper and conduct is contrary to the mind of Christ. Those opinions come not from God that lead to sin.