Matthew 7:15

"Beware of false prophets, for they will come to you in sheep's clothing, but inwardly they are ravening wolves." Behold together with the dogs and swine another kind of ambush and conspiracy, far more grievous than that. For those are acknowledged and open, but these shaded over. For which cause also, while from those He commanded to hold off, these He charged men to watch with exact care, as though it were not possible to see them at the first approach. Wherefore He also said, "beware"; making us more exact to discern them. Then, lest when they had heard that it was narrow and strait, and that they must walk on a way opposite to the many, and must keep themselves from swine and dogs, and together with these from another more wicked kind, even this of wolves; lest, I say, they should sink down at this multitude of vexations, having both to go a way contrary to most men, and therewith again to have such anxiety about these things: He reminded them of what took place in the days of their fathers, by using the term, "false prophets," for then also no less did such things happen. Be not now, I pray you, troubled (so He speaks), for nothing new nor strange is to befall you. Since for all truth the devil is always secretly substituting its appropriate deceit.

What is here spoken of false prophets we may apply to all whose dress and speech promise one thing, and their actions exhibit another. But it is specially to be understood of heretics, who by observing temperance, chastity, and fasting, surround themselves as it were with a garment of sanctity, but inasmuch as their hearts within them are poisoned, they deceive the souls of the more simple brethren.

(Serm. in Mont. ii. 23.) When the Lord had said that there were few that find the strait gate and narrow way, that heretics, who often commend themselves because of the smallness of their numbers, might not here intrude themselves, He straightway subjoins, Take heed of false prophets.

The Lord had before commanded His Apostles, that they should not do their alms, prayers, and fastings before men, as the hypocrites; and that they might know that all these things may be done in hypocrisy, He speaks saying, Take heed of false prophets. What is written below that the Law and the Prophets were until John, (Mat. 11:13.) is said, because there should be no prophecy concerning Christ after He was come. Prophets indeed there have been and are, but not prophesying of Christ, rather interpreting the things which had been prophesied of Christ by the ancients, that is by the doctors of the Churches. For no man can unfold prophetic meaning, but the Spirit of prophecy. The Lord then knowing that there should be false teachers, warns them of divers heresies, saying, Take heed of false prophets. And forasmuch as they would not be manifest Gentiles, but lurk under the Christian name, He said not 'See ye,' but, Take heed. For a thing that is certain is simply seen, or looked upon; but when it is uncertain it is watched or narrowly considered. Also He says Take heed, because it is a sure precaution of security to know him whom you avoid. But this form of warning, Take heed, does not imply that the Devil will introduce heresies against God's will, but by His permission only; but because He would not choose servants without trial, therefore He sends them temptation; and because He would not have them perish through ignorance, He therefore warns them beforehand. Also that no heretical teacher might maintain that He spoke here of Gentile and Jewish teachers and not of them, He adds, who come to yon in sheep's clothing. Christians are called sheep, and the sheep's clothing is a form of Christianity and of feigned religion. And nothing so casts out all good as hypocrisy; for evil that puts on the semblance of good, cannot be provided against, because it is unknown. Again, that the heretic might not allege that He here speaks of the true teachers which were yet sinners, He adds, But inwardly they are ravening wolves. But Catholic teachers should they indeed have been sinners, are spoken of as servants of the flesh, yet not as ravening wolves, because it is not their purpose to destroy Christians. Clearly then it is of heretical teachers that He speaks; for they put on the guise of Christians, to the end they may tear in pieces the Christian with the wicked fangs of seduction. Concerning, such the Apostle speaks, I know that after my departure there will enter among you grievous wolves, not sparing the flock. (Acts 20:29.)

(Mor. xxxi. 14.) Also the hypocrite is restrained by peaceful times of Holy Church, and therefore appears clothed with godliness; but let any trial of faith ensue, straight the wolf ravenous at heart strips himself of his sheep's skin, and shows by persecuting how great his rage against the good.