III. Christians' pious activity has so enraged Satan that he has multiplied heresies to wreak them harm
But at all these acts of godliness, dearly-beloved, which commend us more and more to God, there is no doubt that our enemy, who is so eager and so skilled in harming us, is aroused with keener stings of hatred, that under a false profession of the Christian name he may corrupt those whom he is not allowed to attack with open and bloody persecutions, and for this work he has heretics in his service whom he has led astray from the catholic Faith, subjected to himself, and forced under various errors to serve in his camp.
And as for the deception of primitive man he used the services of a serpent, so to mislead the minds of the upright he has armed these men's tongues with the poison of his falsehoods. But these treacherous designs, dearly beloved, with a shepherd's care, and so far as the Lord vouchsafes His aid, we will defeat. And taking heed lest any of the holy flock should perish, we admonish you with fatherly warnings to keep aloof from the “lying lips” and the “deceitful tongue” from which the prophet asks that his soul should be delivered; because “their words,” as says the blessed Apostle, “do creep as does a gangrene.”
They creep in humbly, they arrest softly, they bind gently, they slay secretly. For they “come,” as the Saviour foretold, “in sheeps' clothing, but inwardly they are ravening wolves;” because they could not deceive the true and simple sheep, unless they covered their bestial rage with the name of Christ. But in them all he is at work who, though he is really the enemy of enlightenment, “transforms himself into an angel of light.” His is the craft which inspires Basilides; his the ingenuity which worked in Marcion; he is the leader under whom Sabellius acted; he the author of Photinus' headlong fall, his the authority and his the spirit which Arius and Eunomius served: in fine under his command and authority the whole herd of such wild beasts has separated from the unity of the Church and severed connection with the Truth.
Source: Sermons (New Advent)