Modern science has determined we don't actually make up our minds about much of anything. By carefully measuring electrical impulses in the brain, scientists have found all the critical decision-making is done before we are consciously aware of it, leaving only a split-second to veto the outcome if we don't like it. Perhaps that is just as well. Consider how long it would take to get though our daily routines if we had to think through every move, starting with brushing our teeth and tying our shoelaces. In fact, it would appear we don't really think through any move, although our minds will often jump in after the fact to provide a ready explanation for our behavior.
As you might image, these latest scientific findings don't sit too well with many philosophers and theologians. These are people who think for a living. They are not happy to discover that our powers of reason are essentially superfluous, at least insofar as most basic decisions about daily living are concerned. The critical issue here is free will. If, in fact, we shuffle through our daily rounds like a bunch of zombies, we are not truly exercising rational judgment or making moral choices.
The implications are profound, although perhaps not as profound as we might think. For instance, if we were to discover that oxygen didn't exist, we wouldn't stop breathing, because breathing is one of those moves we make without really giving it much thought. By the same token, if it turns out free will never existed, we would still go about our business much as before. Would we suddenly give in to every criminal impulse just because free will is an illusion and we are no longer morally responsible for our actions? No, because there are still social constraints on criminal behavior, and we could still wind up in jail. Also, there is growing evidence that empathy and cooperative social behavior may be genetically determined, which is why we feel bad when somebody is harmed by our actions.
The elimination of free will would pose a serious challenge to Christian doctrine, since everything appears to hinge on moral choice. If we do not truly exercise free will, we cannot be held responsible for our actions and therefore cannot sin. Without sin there is no need for salvation and therefore no need for a Savior. However, the implications once again may not be as profound as we might think. Christian theologians have long recognized that human beings are ultimately powerless against sin, which suggests that free will is far from absolute under the best of circumstances, if not merely one of those things that gets invoked after the fact so we can take proper credit for our good behavior.
The issue with sin isn't ultimately about our behavior in any case. Sin arises from the illusion that we exist apart from God, which inevitably engenders a sense that we can exercise a will apart from the will of God. If our existence apart from God is illusory, then free will must also be an illusion. Does this mean our bad behavior is God's will? Only as long as we see things in terms of our will versus God's will. Once we surrender our willfulness, we discover there never was a contest of wills, only the silent unfolding of everything in God's creation from one moment to the next.