Calvin's Theological Determinism

Tonight in my Philosophy class we discussed Free-will and Determinism. That is whether we have freedom of choice or if everything we do is determined for us. From this we branched off into Theological Determinism, whereby our actions are determined by God or deity. One of the proponents of this line of thinking was John Calvin. Calvin's argument for Theological Deteminism goes something like this.
1)If God knows that tomorrow I will do x, then tomorrow I will do x.
2)If tomorrow I will do x, then I cannot not do x.
3)If I cannot not do x, then I have no free choice in the matter.
4)Therefore, if God knows I will do x, then I have no free choice.

In philosophy, to prove an argument invalid you need only find one exception to it. Here is my proposed exception.

If you define x as "disproving God without a shadow of a doubt, then the argument is proven invalid. Let me demonstrate.

1)If God knows that tomorrow I will disprove the existence of God without a shadow of a doubt, then tomorrow I will disprove the existence of God without a shadow of a doubt.
2)If tomorrow I will disprove the existence of God without a shadow of a doubt, then I cannot not disprove the existence of God without a shadow of a doubt.
3)If I cannot not disprove the existence of God without a shadow of a doubt, then I have no free choice in the matter.
4)Therefore, if God knows I will disprove the existence of God without a shadow of a doubt, then I have no free choice.

If God knows I will disprove his existence, then he doesn't exist and can't know anything. Not a belief I hold, but a valid counter argument to Calvin's.