Fair warning from the onset: this one is a bit of a mind-bender. I've recently heard and read some of my hero Sam Harris' work on free will, and I'd like to explore further one consequence of his work which he surprisingly touches on very little, namely the relationship between freewill and the concept of Biblical sin. Sin is of course a central part of the Abrahamic religions. The word appears 441 times in the King James Bible, for example, but all of these can be summarized simply with Romans 6:23, which states that "For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord." This clearly lays out a central tenet of the Christian faith, which is that all people are guilty of sin, and unless you ask for salvation through Jesus, your punishment is eternal damnation. The entire idea of sin is predicated on the assumption that human beings have free will and have the power to make choices. Since we can choose to be good or evil, God brings due rewards or punishment to each person upon death, based on the choices he has made. If it could somehow be demonstrated that human beings do not actually have free will, the entire idea of sin would be fundamentally undermined.
This is precisely what Sam Harris demonstrates in his latest book, Free Will. I would encourage you to read the book or watch one of his lectures on the topic. I'll try to summarize a few of his observations and reasons for asserting that free will is an illusion. Some of these thought experiments get a little crazy, but if nothing else they should provide some very interesting food for thought.
First, try a simple experiment, an obvious act of total free will, of total volition: think of a movie. Any movie at all, even if it's a movie that you haven't even seen but are aware of. What movie did you choose? Now, the important question, why did you choose that movie? If you thought of a few different movies and then ultimately selected one and not the others, can you explain why you didn't pick them? In actual clinical experiments of this sort, people generally give explanations about their choices which turn out to be wrong in reality; a movie pops into their mind, and they try to rationalize why that particular movie occurred to them, and even construct explanations for their choices. Did you pick "The Wizard of Oz?" If you didn't, why not? It's probably because that particular movie didn't even occur to you (if it did, that's a pretty creepy coincidence, and you are probably a sorcerer of some sort.) Are you free to choose a movie which doesn't occur to you? If you can't even really explain why you chose the movie you did, and just as importantly why you decided against other choices which occurred to you, in what sense was your choice actually one of your conscious volition?
Harris is ultimately talking about a kind of determinism, which is not a new idea by any means. The ancient Greeks were all over this, especially the thought that all actions have causes; nothing happens at random, or without cause. This is completely different, however, from the popular Christian sentiment that "everything happens for a reason," which implies that the reason is some sort of divine master-plan. Rather, the idea behind determinism is that every action is the necessary consequence of every action which preceded it; you thought of whatever movie you thought of because of a long string of prior events, such that you actually had no choice but to think of whatever movie you thought of.
Ideas of this sort are actually observable in a lab setting. It is possible to monitor the neural activity of a person and notice the exact moment in the brain when a decision is made. There is inevitably a time lag between the decision at the neural level and the manifestation of the decision as an action by the individual. That is to say, there is a time period in which the brain has made a choice, but the individual isn't actually aware of what that choice is on the conscious level. Sometimes this delay can even be a few seconds, which makes it possible to tell people what they've chosen before they're consciously aware of it themselves.
If you need another example, just consider your thoughts. I mean the random thoughts that just pop into your head throughout the day, for no apparent reason. We all space out sometimes and just start thinking about completely unrelated things, and it is pretty obvious that we aren't in control of this. While we seem to be able to choose to think about whatever we'd like whenever we'd like, we apparently are not the authors of the thoughts which occur to us without our conscious intention.
Another aspect to consider is the role that our genes and upbringing play in shaping who we are, which has a profound effect on our actions. Biologists have discovered various genetic markers which predispose people to certain behaviors as well as certain diseases, and these things are obviously beyond our control. There is a gene linked to psychopathy, for example; people who have this gene are much more likely to engage in violent behavior than those who do not. It is not true, however, that everyone who has this gene becomes a psychopath; environmental factors play an important role as well. So, someone who is genetically predisposed to psychopathic behavior and is brought up in an environment which allows those behaviors to manifest themselves has no choice but to become a violent psychopath. This is simply a result of many prior causes, none of which is the fault of the individual.
This idea is extremely troubling to most people, as our entire justice system is founded on the basic idea that people are responsible for their actions, and it is just to punish people who choose to behave in unacceptable ways. The same statement is true for Christianity and the Biblical notion of sin; if it is not the case that we are the conscious author of our actions, then it is unreasonable that we ultimately be held responsible for them. Thus, the entire idea of "sin" is undermined by the reality that we are not actually in control of our actions. Note equally that this is true of positive actions as well, not simply negative - those who do good deeds are no more the authors of their actions than those who do evil. In light of this observation, it becomes completely unreasonable for God to reward the saved with paradise and punish the sinners with hell, as the individual in either case did nothing except that which had to be done as a consequence of every cause which had preceded, many of which were beyond the individual's control.
Obviously this notion is so completely at odds with our culture that it becomes difficult to believe that it could be true. One is tempted to get carried away with the consequences of it as well, and start wondering if we should release all criminals from prison and stop giving awards to extraordinary people.
This would be utter insanity, of course. Rather, there is a more productive way to use this information as you interact with other people. As you realize that someone who seemingly does intentional harm to you had no choice but to do it, it becomes easier to understand the action and not feel the need for immediate and swift revenge. This does not mean that bad deeds should go unpunished and good deeds uncelebrated. We should not, however, attribute the clearest of conscious intentions to the people who do these things. We should, on the other hand, definitely recognize the completely faulty basis for the concept of sin as a justification for eternal damnation. If God does exist and the Bible is His Word, He has a fundamental misunderstanding of causality and is extremely unjust.
Hopefully this sort of thought process raises a lot of questions; I'd encourage you to watch or read Sam's thoughts on this subject in detail, as he does a better job of explaining and addressing problems with it than I ever could.