It's a tactic of evil that has gone on for millennia. I recently learned from an archaeological study guide that when Jesus' apostles began their ministry after the crucifixion, Hellenists infiltrated the early Church and misled souls into false doctrine (and getting the devout who pushed back arrested). It didn't help that many converts were polytheistic like their ancestors, and changing to monotheism was already a difficult leap for them.

Well, that and a lot of people who call themselves "Christian" don't make Jesus their Lord.