A good tree cannot bring forth evil fruit - Love to God and man is the root of the good tree; and from this principle all its fruit is found. To teach, as some have done, that a state of salvation may be consistent with the greatest crimes, (such as murder and adultery in David), or that the righteous necessarily sin in all their best works, is really to make the good tree bring forth bad fruit, and to give the lie to the Author of eternal truth.

A good tree cannot bring forth evil fruit,.... A man that is unprincipled with the grace of God, has an experimental acquaintance with the Gospel of Christ, and is guided by the Spirit of God into all truth, as it is in Jesus, cannot knowingly deliver, maintain, and abide by any doctrine that is contrary to the glory of God's grace, and the person of Christ, the work of the Spirit, the fundamental doctrines of the Bible; or what is repugnant to the experiences of God's people, and prejudicial to their souls.
Neither can a corrupt tree bring forth good fruit. A corrupt preacher, one destitute of the truth of the Gospel, reprobate concerning the faith, who never had any experience of the doctrines of grace, and denies them in the theory of them, cannot, consistent with himself, and his own principles, deliver, or preach good doctrine; or that which tends to produce any good fruit, either in the experience or lives of men. It is true, a corrupt man, that is, an unregenerate man, may preach sound doctrine, it being what he believes, though he has no experience of it: but then this man is not a corrupt tree, that is, a corrupt preacher, though a corrupt man. As our Lord means by "a good tree", not a good man, barely, or one that is made so by the grace of God; but a good minister, one that is furnished by the Spirit of God, and is well instructed in the kingdom of heaven: so by "a corrupt tree" he does not mean a corrupt man, a man that is in a state of nature, habitually and practically evil; but a corrupt preacher, a false prophet or teacher, that has sucked in corrupt principles, and has nothing else in him, and therefore can bring forth no other.

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.

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.