Assume non-determinism - this is just pure pragmatism because if the universe is conpletely deterministic, what is the point of even this conversation? It would be immoral for god to create us without free will. If we have free will, we must be able to act immorally, else it's not really free will. So god is in a bind - he has to imbue us with the power to go against his wishes, morally speaking. Immorality in humans is a direct consequence of a moral creator.
What Spinoza shows is that, given a set of noncontroversial axioms, free will is essentially undefined. E.g., if you take an action, it is either caused by you, someone/something else, or chance, reasonable assumption? For free will, we would say the action is caused by you. Okay, then why did you take that action? Because of the person you are at that time. Why are you that person? Something at time t-1 made you that person. Either that thing was you, something else, or chance. For the original action at time t to be caused by you, who you are at time t also has to be caused by you, meaning the action at time t-1 that made you who you are also has to be caused by you.
If we follow this recursion far backwards enough, we eventually get to the point where you were a baby, incapable of making your own decisions. This shows that any causal chain can ultimately be traced backwards to something outside "your" control. In which case, how can one sensibly assign responsibility to yourself?
Perhaps another abstract way to put it: Spinoza shows that God creating some kind of free will that made actions unpredictable to him is equivalent to the idea of God creating a rock so heavy that he himself cannot lift; it's incompatible with the idea of omnipotence. Undefined in the sense that the set of all sets that are not members of themselves is undefined.
