Oddly, if an all powerful creator being, separate from its creation, maintains almighty control of creation (Master Plan) - free will does not exist.
If, on the other hand, the level of unpredictably (or pseudo-chaos) is so high as to at least approach an asymptote of actual chaos, then virtual free will - with no insubstantial interactions* (supernatural events) - could be indistinguishable from actual free will.
I think that, just because religions are fond of positing the existence of free will (frankly, as a means of exonerating their god from the 'sins' of its creation) not only does not, necessarily tie it to insubstantial interaction - but makes it impossible in this particular model of substantive dualism.
I am not actually claiming that free will even exists at this high level of virtuality. But I wonder how a monotheist can buy into it within the framework of their belief in a Master Plan while an atheist can insist that its non-existence is actually pertinent and must be supernatural.
Keep in mind, a computer cannot generate an actual random number. It may be nearly impossible to work backward from the string to the algorithm that generated it - but that algorithm is there and is known. Whereas, chaos theory has shown that very simple equations can generate highly unpredictable results in their iterations due to a minute displacement of the 'starting point.' Thus, the so-called butterfly effect.
Therefore, one fairly valid response to the statement 'free will does not exist' is 'so what?"
*insubstantial interaction (supernatural) - anything without substance that, nevertheless, is said to interact with substantive reality. For example, if a ghost is not a physical thing (composed of atoms, etc.) then how could it be detected with the senses at all, since senses respond to substantive stimuli?