Beware of false prophets, who come to you in sheep's clothing, but inwardly they are ravening wolves.
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Augustine of Hippo
Here, therefore, those who promise a wisdom and a knowledge of the truth which they do not possess, are especially to be guarded against; as, for instance, heretics, who frequently commend themselves on account of their fewness. And hence, when He had said that there are few who find the strait gate and the narrow way, lest they [the heretics] should falsely substitute themselves under the pretext of their fewness.
In the clothing of sheep. Beware of hypocrites, with their outward appearance of sanctity, and sound doctrine
by their fruits you shall know them. Such hypocrites can scarcely ever continue constant in the practice of what is good. (Witham)
Heretics usually affect an extraordinary appearance of zeal and holiness, calling themselves evangelical preachers and teachers of the gospel, as if that Church which preceded them, and which descends by an uninterrupted succession from the apostles, did not teach the pure gospel of Christ. (Haydock)
Beware of false prophets, or heretics. They are far more dangerous than the Jews, who being rejected by the apostles, are also avoided by Christians, but these having the appearance of Christianity, having churches, sacraments deceive many. These are the rapacious wolves, of whom St. Paul speaks, Acts xx. (St. Chrysostom, hom. xix.) Origen styles them, the gates of death, and the path to hell. (Com. in Job. lib. i. Tom. 2.)
Notice that along with the dogs and pigs there is another form of ambush and plotting that causes far more trouble than that of dogs or pigs. For the dogs and pigs do not hide their nature. They are obvious. But the movements of wolves are hidden in shadows. Hence he commanded that one should not only avoid the dogs and pigs but also that one should continuously and carefully look out for wolves, because one cannot see them the moment they attack. For this reason he says, “Watch out!”—to make his hearers more careful to distinguish deception from truth. The Gospel of Matthew, Homily
Jesus reminded them of what happened to their ancestors who were attracted to false prophets. The same dangers are now faced as those that occurred in earlier days. He reminded them of the experience of their ancestors so that they would not despair at the multitude of troubles that would mount up on this way that is narrow and constricted. He reminded them that it is necessary to walk in a way that goes contrary to the common opinion. One must guard oneself not only against pigs and dogs but those other, more elusive creatures: the wolves. They were going to face inward anxieties as well as outward difficulties, but they are not to despair. “Therefore do not be thrown into confusion,” Jesus says in effect, “for nothing will happen that is new or strange. Remember that the ancient adversary is forever introducing deception as if true.” The Gospel of Matthew, Homily
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 yo...