The Enchiridion by St. Augustine
Chapter 60.--It is More Necessary to Be Able to Detect the Wiles of Satan When He Transforms Himself into an Angel of Light.
It is more necessary to use all our powers of discrimination and judgment when Satan transforms himself into an angel of light, lest by his wiles he should lead us astray into hurtful courses. For, while he only deceives the bodily senses, and does not pervert the mind from that true and sound judgment which enables a man to lead a life of faith, there is no danger to religion; or if, feigning himself to be good, he does or says the things that befit good angels, and we believe him to be good, the error is not one that is hurtful or dangerous to Christian faith. But when, through these means, which are alien to his nature, he goes on to lead us into courses of his own, then great watchfulness is necessary to detect, and refuse to follow, him. But how many men are fit to evade all his deadly wiles, unless God restrains and watches over them? The very difficulty of the matter, however, is useful in this respect, that it prevents men from trusting in themselves or in one another, and leads all to place their confidence in God alone. And certainly no pious man can doubt that this is most expedient for us.