Apologies in advance. This post may be incoherent. I tried to simplify this incredibly complicated idea that I can only just grasp. I do recommend the Reasonable Doubts podcast for further clarification if you are interested in further discussion.
As I have mentioned before, when I was a Christian, my theology/philosophy (theological philosophy?) was Calvinism. I liked the cold, cruel logic of predestination. It wasn't very comforting and didn't reflect much mercy or love from god, but it made perfect sense. The fact that the pieces fit together without too many gaps was what kept me calling myself a Christian for so long.
The funny thing is, now that I am an atheist, I have roughly the same views. That is, I am what you might call a determinist. One of the main objections to both determinism and Calvinism is that the popular concept of free will is compromised. For those who are unfamiliar with determinism, the basic idea is that one thing causes another - think of a domino falling into another. Then just expand that idea - think of a Rube Goldberg device. Even if it doesn't work, the sequence of events was inevitable and, given all the exact conditions (including the atmosphere, temperature, etc) it could never happen any other way. So then take this idea and put it into the realm of our minds. Things that seem to be decisions we make are really inevitabilities based on the physical properties of our brains, and the physical form of our brains are formed by a seemingly endless trail of cause and effect. Just as the apparently random path of a water droplet on a window is actually inevitable, predetermined to run its course by gravity, the angle of the window, defects of the glass and grime on the surface, so too is our own life a series of dominoes in the middle of an inescapable pattern.
Where people run into problems is that they assume that all of this means that they can't change their lives. In one sense, of course, this is true. But from a practical standpoint it is not. Determinism is not a practical philosophy. It is primarily a mind game with limited (although real) philosophical benefits, which I will address in a bit. One common reaction to determinism is to just throw up your hands and wonder why you even bother - if everything is already determined, then what is the point of trying to improve your life? Determinism is very different from the ideas of fate and destiny.
The best thing to do is to look at the past because all of those decisions have already been made. And I do use the language of decision making because it describes the mental process that we go through, even if the outcome is not really up to us. So look at the past - think about where you are right now in your life. Then think back to the decisions that led you there. It rapidly blooms into a web far to dense and complicated to follow. You think about the interactions between yourself and other people, or objects that were formed or moved by people and then the people before those people whose decisions led to the way things are today. If things were only slightly different fifty years ago, certain people would not have met when they did, which would change the sex partners or even just the conception date which would change the offspring produced. The apparent possibilities are staggering - infinite even - but they aren't really possibilities. Every decision, no matter how nuanced, has a single outcome.
A simple example is lunch. What are you going to have for lunch? That depends on where you are going to go. Although you may have as many as a dozen potential restaurants to choose from, your eventual decision will rely information that is already in your head: images you have seen on your way to work, advertisements, the physical location of the restaurants and the comparative difficulty of getting to one versus another, your history at one restaurant, the prices of the food. Although there are truly many paths, only one will be taken and that was the inevitable one. So we still have to make the decision, but it is really just our brain clinking through the gears and dominoes until it gets to the answer.
As I said, it is a mental game and should have no real impact on your approach to problems. Where the philosophy does have merit and value is in understanding our roles in the world. As with Calvinism, determinism devalues and humbles all of us (although a stark difference is that there is no god of value) to the point where we are all equal. All living creatures are on rails of iron, intersecting with each other millions of times a second.
It is tempting to end the process there and say that because we are all stuck in our eventual paths then there's no need to be kind or generous or caring. But that's a false assumption to make. We can't see the mechanisms of our mind, so it is best to act as though we are making decisions. Make good decisions and remember that your actions influence the eventual decisions of others. So what if free will is ultimately an illusion? Where I eat for lunch is going to be, after everything has been taken into account, the place where I most want to eat.