Matthew 7:17

So every good tree - As the thorn can only produce thorns, not grapes; and the thistle, not figs, but prickles; so an unregenerate heart will produce fruits of degeneracy. As we perfectly know that a good tree will not produce bad fruit, and the bad tree will not, cannot produce good fruit, so we know that the profession of godliness, while the life is ungodly, is imposture, hypocrisy, and deceit. A man cannot be a saint and a sinner at the same time. Let us remember, that as the good tree means a good heart, and the good fruit, a holy life, and that every heart is naturally vicious; so there is none but God who can pluck up the vicious tree, create a good heart, plant, cultivate, water, and make it continually fruitful in righteousness and true holiness.

A corrupt tree - The word "corrupt" here does not signify, as our translation would seem to indicate, that the tree "had been" good, but had become "vitiated;" but that it was a tree of a useless character, of a nature that produced nothing beneficial.