17 “So every good tree bears good fruit, but the bad tree bears bad fruit.
18 “A good tree cannot produce bad fruit, nor can a bad tree produce good fruit.
19 “Every tree that does not bear good fruit is cut down and thrown into the fire.
20 “So then, you will know them by their fruits.” Matthew 7:15-20.
Here are some quotes (not contiguous) from Calvin’s Commentary on these verses:
“It was a general expectation among the Jews that, under the reign of Christ, their condition would be delightful, and free from all contest or uneasiness. He therefore warns his disciples that, if they desire to persevere, they must prepare themselves to avoid the snares of Satan. It is the will of the Lord, (as has been already said,) that his Church shall be engaged in uninterrupted war in this world. That we may continue to be his disciples to the end, it is not enough that we are merely submissive, and allow ourselves to be governed by his Word. Our faith, which is constantly attacked by Satan, must be prepared to resist.
(P.S. Professor Hanko – This paragraph above reminds me very much of what you said to me before. 
It is of the greatest consequence, undoubtedly, that we should suffer ourselves to be directed by good and faithful ministers of Christ.
“The sheep hear the voice of the shepherd; and a stranger they do not follow, but flee from him,” (John 10:3, 5). Hence too we infer, that there is no reason why believers should be discouraged or alarmed, when wolves creep into the fold of Christ, when false prophets endeavor to corrupt the purity of the faith by false doctrines. They ought rather to be aroused to keep watch: for it is not without reason that Christ enjoins them to be on their guard. Provided that we are not led astray through our own sluggishness, we shall be able to avoid every kind of snares; and, indeed, without this confidence, we would not have the courage necessary for being on our guard. Now that we know that the Lord will not fail to perform his promises, whatever may be the attacks of Satan, let us go boldly to the Lord, asking from him the Spirit of wisdom, by whose influences he not only seals on our hearts the belief of his truth, but exposes the tricks and impositions of Satan, that we may not be deceived by them.
From their fruits you shall know them. Had not this mark of distinction been added, we might have called in question the authority of all teachers without exception. If there is a mortal danger to be dreaded in teachers, and if we see no way of avoiding it, we shall be under the necessity of holding them all suspected: and there will be no better or shorter method than to keep our ears shut against them all. We see that ungodly men, to screen themselves when rejecting every kind of doctrine, hold out this danger, and that weak and ill-informed persons remain in a state of perplexity. That our reverence for the Gospel, and for its faithful ministers and teachers, may not be diminished, Christ enjoins us to form our opinion of the false prophets from their fruits.
Under the fruits the manner of teaching is itself included, and indeed holds the chief place: for Christ proves that he was sent by God from this consideration, that “he seeketh not his own glory, but the glory of the Father who sent him,” (John 7:18.)
Do men gather grapes from thorns, or figs from thistles? By these proverbs, which were then in common use and universally received, Christ confirms his statement, that no man can be deceived by false prophets, unless he is wilfully blind: for the fruits as plainly discover upright servants of God, and unfaithful workmen, as the fruits point out the nature of the tree.”
Here is the link to the full Commentary by Calvin on these verses –
Reformed Doctrine Daily Devotions 