Chapter 7:15-23 A Tree and Its Fruit

7: 15 Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.
Beware of false prophets - teachers of erroneous doctrines, who come professing a commission from God, but whose aim is not to bring the heavenly treasure to the people, but rather to rob them of their earthly good. 

In sheep's clothing - With outside religion and fair professions of love: 

Wolves - Not feeding, but destroying souls.

7:16 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
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.

7:17 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.

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. 

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.

7:18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.

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.

7:19 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.

How dreadful then is the condition of that teacher who hath brought no sinners to God!

7:20 Wherefore by their fruits ye shall know them.

This is the conclusion of the whole, and a repetition of what is before said, the more to fix the rule of judgment upon their minds, and engage them to try men by their doctrines, and their doctrines by the standard of the Scriptures, and not believe every spirit; for with some care and diligence such persons may be detected, and the malignant influence of their ministry be prevented.

7:21 Not every one that saith unto Me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of My Father which is in heaven.

It is an answer to that question, Psalm 15:1. Who shall sojourn in thy tabernacle? - the church militant; and who shall dwell in thy holy hill? - the church triumphant.

7:22 Many will say to Me in that day, Lord, Lord, have we not prophesied in Thy name? and in Thy name have cast out devils? and in Thy name done many wonderful works?

We have prophesied - We have declared the mysteries of Thy kingdom, wrote books; preached excellent sermons: In Thy name done many wonderful works - So that even the working of miracles is no proof that a man has saving faith.

7:23 And then will I profess unto them, I never knew you: depart from Me, ye that work iniquity.

Then will I profess unto them - Publicly before men and angels, at the day of judgment.
I never knew you - which must be understood consistent with the omniscience of Christ; for as the omniscient God He knew their persons and their works, and that they were workers of iniquity; He knew what they had been doing all their days under the guise of religion; He knew the principles of all their actions, and the views they had in all they did; nothing is hid from Him.
Depart from Me - What a terrible word! What a dreadful separation!