2 Corinthians 11:14

"And no marvel; for if even Satan fashioneth himself into an angel of light, is it a great thing if his ministers also fashion themselves as ministers of righteousness?" So that if one ought to marvel, this is what he ought to marvel at, and not at their transformation. For when their teacher dares do any thing, no marvel that the disciples also follow. But what is "an angel of light?" That hath free liberty to speak, that standeth near to God. For there are also angels of darkness; those which be the devil's, those dark and cruel ones. And the devil hath deceived many so, fashioning himself "into," not becoming, "an angel of light." So do also do these bear about them the form of an Apostle, not the power itself, for this they cannot. But nothing is so like the devil as to do things for display. But what is "a ministry of righteousness?" That which we are who preach to you a Gospel having righteousness. For he either means this, or else that they invest themselves with the character of righteous men. How then shall we know them? "By their works," as Christ said. Wherefore he is compelled to place his own good deeds and their wickedness side by side, that the spurious may become evident by the comparison.

These illusions are apparitions of that spirit who seeks to ensnare unhappy souls in the deceptive rites of a multitude of false gods and to turn them aside from the true worship of the true God, by whom alone they can be purified and healed.

For so great is the influence of probity and chastity, that all men, or almost all men, are moved by the praise of these virtues; nor is any man so depraved by vice, but he hath some feeling of honor left in him. So that, unless the devil sometimes transformed himself, as Scripture says, into an angel of light, he could not compass his deceitful purpose. Accordingly, in public, a bold impurity fills the ear of the people with noisy clamor; in private, a feigned chastity speaks in scarce audible whispers to a few: an open stage is provided for shameful things, but on the praiseworthy the curtain falls: grace hides, disgrace flaunts: a wicked deed draws an overflowing house, a virtuous speech finds scarce a hearer, as though purity were to be blushed at, impurity boasted of. Where else can such confusion reign, but in devils' temples? Where, but in the haunts of deceit?

Satan sometimes transforms himself into an angel of light in order to test those who need testing or to deceive those who deserve deception. Nothing but the great mercy of God can save a man from mistaking bad demons for good angels and false friends for true ones and from suffering the full damages of this diabolical deception which is all the more deadly in that it is wily beyond words.