I think you're giving them more intelligence than they deserve. If you fake convert, and say everything they want you to say, they should be fooled. If you're really clever about it, you can start asking the right questions and have them start doubting their own religion.

"Let me get this straight. Mohammed said he talked to God, and God said that God would never talk to anyone else ever again? I spoke to God and he said that he never told Mohammed that. How do you know Mohammed wasn't just telling people what he thought they wanted to hear?"

"Your religion says that lying to non-believers is allowed? You're also allowed to lie pretty much whenever you think you can gain an advantage? How do you know that your religious leaders aren't lying to you, if your religion explicitly says that lying is allowed?"