2 Corinthians 11:14
And no marvel; for Satan himself is transformed into an angel of light.
Note 23 at 2Co 11:14: Paul didn't think it strange that Satan's ministers would try to imitate godly people. That's the way Satan is himself--like father, like son. The serpent didn't come to Eve in the Garden, threatening to bite her if she didn't eat of the forbidden fruit. Instead, he came with deception, presenting himself as being concerned for her welfare. Satan's greatest weapon is deceit. For this reason, Christians must "try the spirits whether they are of God" (1Jo 4:1).