A Good Tree Cannot Bear Bad Fruit

The illustration of this comparison, of the fruit's being the discovery of the tree. You cannot always distinguish them by their bark and leaves, nor by the spreading of their boughs, but by their fruits ye shall know them. The fruit is according to the tree. Men may, in their professions, put a force upon their nature, and contradict their inward principles, but the stream and bent of their practices will agree with them. Christ insists upon this, the agreeableness between the fruit and the tree, which is such as that, If you know what the tree is, you may know what fruit to expect. Never look to gather grapes from thorns, nor figs from thistles; it is not in their nature to produce such fruits. An apple may be stuck, or a bunch of grapes may hang, upon a thorn; so may a good truth, a good word or action, be found in a bad man, but you may be sure it never grew there. Note,

Corrupt, vicious, unsanctified hearts are like thorns and thistles, which came in with sin, are worthless, vexing, and for the fire at last.
Good works are good fruit, like grapes and figs, pleasing to God and profitable to men.
This good fruit is never to be expected from bad men, and more than a clean thing out of an unclean: they want an influencing acceptable principle. Out of an evil treasure will be brought forth evil things.

On the other hand, if you know what the fruit is, you may, by that, perceive what the tree is. A good tree cannot bring forth evil fruit; and a corrupt tree cannot bring forth good fruit, nay, it cannot but bring forth evil fruit. But then that must be reckoned the fruit of the tree which it brings forth naturally and which is its genuine product-which it brings forth plentifully and constantly and which is its usual product. Men are known, not by particular acts, but by the course and tenour of their conversation, and by the more frequent acts, especially those that appear to be free, and most their own, and least under the influence of external motives and inducements. - Matthew Henry