I believe John Calvin has a better commentary on these words of Christ than I could ever pen down. Therefore, I will let him speak.
“But beware of false prophets (Matthew 7:15). These words were intended to teach, that the Church would be exposed to various (impostors), and that consequently many would be in danger of falling from the faith, if they were not carefully on their guard. We know what a strong inclination men have towards falsehood, so that they not only have a natural desire to be deceived, but each individual appears to be ingenious in deceiving himself.
Satan, who is a wonderful contriver of delusions, is constantly laying snares to entrap ignorant and heedless persons. (Jesus) therefore warns his disciples that, if they desire to persevere (in the faith), 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.
It is of the greatest consequence, undoubtedly, that we should let ourselves to be directed by good and faithful ministers of Christ: but as false teachers, on the other hand, make their appearance, if we do not carefully watch, and if we are not fortified by perseverance, we shall be easily carried off from the flock. To this purpose also is that saying 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 conclude, 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. When Christ says, that they come to us in sheep’s clothing, but inwardly are ravening wolves, his meaning is, that they (will seem to be true), if prudence be not exercised in subjecting them to a thorough examination” ~ John Calvin (July 10, 1509 to May 27, 1564)