2 Corinthians 11:13-15 (NIV)
13For such men are false apostles, deceitful workmen, masquerading as apostles of Christ.
14And no wonder, for Satan himself masquerades as an angel of light.
15It is not surprising, then, if his servants masquerade as servants of righteousness. Their end will be what their actions deserve.
Titus 1:16 (NIV)
16They claim to know God, but by their actions they deny him. They are detestable, disobedient and unfit for doing anything good.
Matthew 7:15 (NIV)
15“Watch out for false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves.