Some people argue that if God exists, there cannot exist free will. God is omniscient. He knows everything.
Say that tomorrow for breakfast you will choose between toast or a banana. God knows you will choose the banana, because he knows every single thing. If instead you choose toast, then God would have been wrong, and thus not omniscient, which he is. If God knows you are going to choose the banana, then there is no possibility of you choosing toast, thus you don't have free will.
There are two problems with this argument.
1. It confuses God's foreknowledge with necessity. Take the last sentence. If God (or anyone else) knows that that you are going to choose a banana for breakfast, it does not necessarily follow that you lack free will. That's because your decision to eat a banana is completely isolated from God's knowledge. Unless you consulted God ahead of time, God's foreknowledge did not affect your decision; therefore, it took nothing away from the decision.
Let's use a non-divine example. I know with 100% certainty that the US Congress is not going to have a balanced budget this year, or next year. My knowledge of this in no way hinders the free will of the congressmen.
2. God necessarily exists outside of time. From my perspective, "what happens tomorrow" is knowledge of the future. To God, it is knowledge of the present, because he is equally present then as he is now.
I kind of breezed through the argument; there is honestly a lot more that can be said about this. A really thorough answer would explain about the nature of time, knowledge and free will. When all is said and done, though, the possibility of free will not existing is merely a thought experiment. Our current society is formed around the idea that free will exists, and people are responsible for making their own choices. America is a pro-choice society, in more ways than one.