1 Vice mimics virtue, and the tares strive to be thought wheat, growing like the wheat in appearance, but being detected by good judges from the taste. The devil also transfigures himself into an angel of light; not that he may reascend to where he was, for having made his heart hard as an anvil, he has henceforth a will that cannot repent; but in order that he may envelope those who are living an Angelic life in a mist of blindness, and a pestilent condition of unbelief.
Many wolves are going about in sheeps' clothing, their clothing being that of sheep, not so their claws and teeth: but clad in their soft skin, and deceiving the innocent by their appearance, they shed upon them from their fangs the destructive poison of ungodliness. We have need therefore of divine grace, and of a sober mind, and of eyes that see, lest from eating tares as wheat we suffer harm from ignorance, and lest from taking the wolf to be a sheep we become his prey, and from supposing the destroying Devil to be a beneficent Angel we be devoured: for, as the Scripture says, he goes about as a roaring lion, seeking whom he may devour. This is the cause of the Church's admonitions, the cause of the present instructions, and of the lessons which are read.
Source: Catechetical Lectures (New Advent)