If indeed there is such a thing as free will then by definition it must be supernatural. If, on the other hand, all of our actions are determined by a intricately complex set of natural causes and effects then our power of decision is merely an illusion. How can we tell if free will exists? Simple: We all make mistakes. Programmed automatons can’t, you know, no matter how sophisticated. So-called “computer-error” is actually user error, programmer error, manufacturer error or environmental causes outside the computer’s control. How do I know I am right about all this? Because I can’t be in error without proving myself correct. If I am a programmed automaton and it is not my error, then whose is it? Free will is necessary to make a mistake.
In any theological struggle, the first thousand years are always the bitterest.