It is my belief that the concept of free will makes no sense. This has nothing to do with science or theology, but the fact that I think "free will" is self-contradictory, in a sense.
What reason and/or cause is there for one to make whatever decision they make? If there is no reason/cause, then the decision is arbitrary and/or spontaneous, and not truly "willed" any more than a hiccup is "willed". If there is a reason/cause, then the decision is contingent on other factors, and not truly "free". Now at this point someone might say something along the lines of "well, there may have been a reason/cause, but it was ignored, or disregarded, or etc." But that just leads to a regress: what reason/cause was there then for the ignorance (or whatever) (once, again, if there was no reason/cause, then the ignorance was arbitrary and random)?
Let's illustrate this by considering a simple case: someone has to make a choice between two options, which we'll call "good" and "bad", however those terms are defined (they are essentially arbitrary designations in this argument). Let's consider the situation where someone makes a "good" choice. Assuming that the choice wasn't arbitrary or random, what was the reason/cause that that choice was made? The answer "because it was the good choice" makes no sense, because if that is the sole cause for making the choice, then everyone would make the "good" choice, which destroys the concept of free will, since every decision would have only one absolute outcome. Neither does the answer "because the person is good" make any sense, because it implies that the choices made by an individual are determined by the individual's nature. Is it impossible for someone of a "good" nature to make a "bad" decision? If so, then the person is constrained by their nature, and are not free. If not, then why would someone with a "good" nature make a "bad" choice, if not for some other reason/cause? And once again, if one was to argue that there is a reason/cause but it may or may not influence the decision, the question becomes: well, what was the reason/cause for whether or not the decision was influenced? And you have a infinite regress.
That is essentially my argument for why "free will" conceptually makes no sense. Practically, I also think even if free will exists, the opportunities one has to practice it are so few as to make it effectively impossible. Each decision one makes is dependent on a number of external factors: namely, what options are available. Among "options" I'm also including what is permitted by one's mindset, psychology, neurology, etc. These options are what they are because of numerous miniscule decisions made in the past, which were too tiny to be noticed (and as such are effectively meaningless as far as free will is concerned), but are amplified over time by the "butterfly effect", eventually determining what large-scale options we have available.
Now, someone might say "well, God gives us free will", but this really just makes the problem worse. First, it does not solve the problem of free will itself being self-contradictory. Secondly, it is usually assumed that God is omniscient. If we define omniscient to mean it knows EXACTLY what will happen at any point in the future, free will makes no sense. In order for us to have free will, we must have choice. In order to have choice, we must have a number, more than one, of practicable options. But if god is omniscient in the sense described above, there is only one practicable option. Notice this says nothing about whether or not God CONTROLS our actions: the mere fact that is knows our actions in advance violates free will. And in traditional theology, this is a major problem, since God judges those decisions and determines punishment and reward for them. I read something once where an apologist was arguing about how materialism must imply determinism, and they saw this a problem, yet I would argue that at least in materialism there is no entity determining ultimate reward or punishment for actions we cannot control.
Part 2 will be about the implications of the lack of free will, which I believe are much less dire than people seem to think. (Essentially, my arguments rest on the idea that we do not have to worry about people acting as if they have no free will, since the very statement "acting as if they have no free will" implies a willed decision to act a certain way.)