2 Corinthians 11:14-15
“And no marvel; for if even Satan fashions 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 has free liberty to speak, that stands near to God. For there are also angels of darkness; those which be the devil's, those dark and cruel ones. And the devil has 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. And when about again to enter upon his own praises, he first accuses them, in order to show that such an argument was forced upon him, lest any should accuse him for speaking about himself, and says,
Source: Homilies on Second Corinthians (New Advent)