Read Brannon Howse's column on this topic, here. Or listen to Howse's radio show about David Barton and Glenn Beck.
"Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness? And what accord has Christ with Belial? Or what part has a believer with an unbeliever? And what agreement has the temple of God with idols?" 1 Corinthians 6:14-16a
"But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed." Galatians 1:8-9