I know many Christians believe they have free will, but they also believe their God is omniscient. Omniscience means he knows everything--past, present and future.

So, there lies a problem. If he knows everything from now until the end, how can you believe there's free will then? If God knows you're going to do X can you do Y instead? No, not if he's truly omniscient. If you truly believe God is omniscient then you can not believe you have free will. They contradict one another.

That contradiction brings up the problem that either God isn't omniscient or you don't have free will.