The test our Lord gives for "discerning the spirits" is one which requires time for its application, but it is the only sure one; and when we remember that the Master is now looking forward into the future history of His kingdom, we can see why He should lay stress on a test whose operation, though slow, was sure. It is of course assumed that the first criterion is the Word of the Lord Himself. This is the law of the kingdom; but, knowing well what was in man, the Lord could not but foresee that there would be those who could so twist any words that might be spoken on those great subjects as to lay snares for the unwary; and therefore, besides the obvious appeal "to the law and to the testimony," He supplied a practical test which, though less speedy in its application, was perfectly sure in its results.
The announcement of so important a test leads to the development of the general principle on which its validity depends--viz., the vital connection between essential doctrine and life. In the long run the one is always the outcome of the other. In the spiritual as in the natural world every species brings forth fruit "after its kind." "Do men gather grapes of thorns, or figs of thistles? Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit." The law being so absolute, making it certain, on the one hand, that where there is truth in the inward parts there will be good fruit in the outward life, and on the other, that where there is corrupt fruit in the outward life there must be that which is corrupt in the hidden man of the heart, it follows that the criterion is so sure as to be without appeal: "Every tree that bringeth not forth good fruit is hewn down, and cast into the fire" (ver. 19), and therefore may well determine the question as to who are trustworthy teachers in the Church: "Wherefore by their fruits ye shall know them."
In the development of the principle the Master's thought has been enlarged so as to include not teachers only, but all His disciples; and His range of view has been extended so as to embrace the last things. The great day of Judgment is before him. He sees the multitudes gathered around the throne. He foresees that there will be many on that great day who will discover, when it is too late, that they have allowed themselves to be deceived, that they have not been careful enough to test their spiritual guides, that they have not been careful enough to try themselves and make sure that their fruits were such that the Lord of the vineyard could recognise them as His own. He is filled with sympathy and sorrow at the prospect; so He lifts up His voice in earnest warning, that, if possible, none of those to whom the words will ever come may allow themselves to fall into so fatal an error: "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."
How naturally, and as it were unconsciously and inevitably, He has passed from the Teacher to the Judge! Not as a personal claim. In His earliest teaching He kept personal claims as much in the background as possible. But now it is impossible to avoid some disclosure of His divine authority. He must speak of the Judgment; and He cannot speak of it without making it appear that He is Judge. The force of this is all the greater that He is, as it were, surprised into it; for He is evidently not thinking of Himself at all, but only of those who then were or would afterwards be in danger of making a most fatal mistake, leading to consequences awful and irreparable. We can well imagine that from this point on to the end there must have been a light on His face, a fire in His eye, a solemnity in His tone, a grandeur in His very attitude, which struck the multitude with amazement, especially at the _authority_ (ver. 29) with which He spoke: "Many will say to Me in that day, Lord, Lord, did we not prophesy by Thy name, and by Thy name cast out devils, and by Thy name do many mighty works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity" (R.V.).