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
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.