Albert Barnes Commentary 2 Corinthians 11:14

"And no marvel; for even Satan fashioneth himself into an angel of light." — 2 Corinthians 11:14 (ASV)

And no marvel. And it is not wonderful (2 Corinthians 11:15). Since Satan himself is capable of appearing to be an angel of light, it is not to be considered strange that those who are in his service also should resemble him.

For Satan himself is transformed, etc. That is, he who is an apostate angel, who is malignant and wicked, who is the prince of evil, assumes the appearance of a holy angel. Paul assumes this as an indisputable and admitted truth, without attempting to prove it and without referring to any particular instances.

Probably Paul had in mind cases where Satan took on false and delusive appearances for the purpose of deceiving, or where he assumed the appearance of great sanctity and reverence for the authority of God. Such instances occurred in the temptation of our first parents (Genesis 3:1–6) and in the temptation of the Savior (Matthew 4).

The phrase "an angel of light" means a pure and holy angel—light being the emblem of purity and holiness. Such are all the angels that dwell in heaven, and the idea is that Satan assumes such a form as to appear to be such an angel.

Learn here:

1. His power. He can assume such an aspect as he pleases. He can dissemble and appear to be eminently pious. He is the prince of duplicity as well as of wickedness; and it is the consummation of evil power for an individual to be able to assume any character he pleases.
2. His art. He is long practiced in deceitful arts. For six thousand years he has been practicing the art of delusion, and with him it is perfect.
3. We are not to suppose that all that appears to be piety is piety. Some of the most plausible appearances of piety are assumed by Satan and his ministers. No one ever professed a profounder regard for the authority of God than Satan did when he tempted the Savior. And if the prince of wickedness can appear to be an angel of light, we are not to be surprised if those who have the blackest hearts appear to be men of most eminent piety.
4. We should be on our guard. We should not listen to suggestions merely because they appear to come from a pious man, nor because they seem to be prompted by a regard to the will of God. We may be always sure that if we are to be tempted, it will be by someone having a great appearance of virtue and religion.
5. We are not to expect that Satan will appear to man to be as bad as he is. He never shows himself openly to be a spirit of pure wickedness, or black and abominable in his character, or full of evil and hateful. He would thus defeat himself. It is for this reason that wicked men do not believe that there is such a being as Satan. Though continually under his influence and led captive by him at his will, yet they neither see him nor the chains which lead them, nor are they willing to believe in the existence of the one or the other.

* "marvel": wonder. a. "Satan himself": Genesis 3:1, 5; Revelation 12:9.