I'm not talking about "free will" in the philosophical sense. This is about the option that believers claim we have when it comes to whether or not to believe in God, or whatever god they happen to be discussing with us.
According to the tenets of Judeo-Christian religions, God, the father, is omnipotent and naturally it follows that if Jesus and the Holy Ghost are part of the God-trinity, this applies to them as well. Thus, God, Jesus and the Holy Ghost are omni-everything: omnipotent, omniscient, omnipresent and so on, in a word, omnifarious. To put it simply, there is nothing that God cannot do.
By saying this, the believer then accepts that this god is able to know, see and not only do anything, but also to affect the outcome of everything.
If this is the case, then there are a few questions that arise.
If God was able to foresee, and if he is omni-everything, then he can also foresee the future, he knew that Adam and Eve were going to eat the fruit of the tree, whether or not he told them not to. He also knew that they would be tempted by the snake, and take the snake's word for it about eating the fruit of the tree, and he knew that they would discover their nakedness. So then why did he set them up for failure? Believers claim that he "also gave them free will to choose to not eat the fruit." But if they had chosen to ignore the snake and not eat the fruit, then he already knew they would do that. And then what would have been the point of the story? So there was no free will involved: they did exactly what God wanted them to do.
This also applies to everything else that happens, God knows everything, and the way everything is going to turn out, because he can see, and control everything. So why does he sit back and do nothing when people make stupid choices? And if he does, then he's just playing with them, and if they're doing what he knew they were going to do and if he arranged everything for them to do what they do, there's no free will involved.
According to this belief then, that I don't believe in God, is not my choice. God is controlling what I do and what choices I make, so I'm doing what he set me up to do. He chose me for a future in an eternal hellfire by giving me a brain that can reason that he is merely a myth created by Near Eastern desert dwellers, but, if he exists, he still chose to create me and every situation in which I find myself, purely for his own amusement.
Thus, everything that's happened everywhere, ever since the "creation" has happened because God made it happen, you can't have free will and still be subject to his omnifariousness. If we have free will, then God is not Almighty, and therefore omnipotent. If he had to search for Adam and Ever, he didn't know where they were, he is not omnipresent and omniscient, and his saying that he is, is a lie.
Therefore, if we accept that we have free will, then God is impotent. And if he doesn't know what we are going to choose to do, because we have "free will" to choose which road we're going to take, then he is also not aware of the future. And by this deduction, he is not really a god, is he?