Ezequiel 9

Ezequiel 9:4

Thus [the devil] suffers and is dishonored; and although he still ventures with shameless confidence to disguise himself, yet now, wretched spirit, he is detected rather by those who bear the sign on their foreheads; and he is even rejected by them, and is humbled and put to shame. For even if, now that he is a creeping serpent, he shall transform himself into an angel of light, yet his deception will not profit him; for we have been taught that "though an angel from heaven preach to us any other gospel than that we have received, he is anathema." — LETTER TO THE BISHOPS OF EGYPT 1:2