Since most of my interaction with religion has been with Christianity, a lot of the time when I refer to God, I’m speaking of the Christian one. Essentially though, my arguments work against all religious beliefs.
The first reason that I did not believe in God was a rather simple one, that I formulated all by myself at around 7 years old.
Given that heaven is often depicted (I don’t think I used the word depicted) as being in the sky, and we’ve sent rocket ships through the sky to the moon, surely they should have run into heaven somewhere along the way?
It was a reasonable question given my interaction with Christianity at the time (i.e a simplistic, childish level) and it was probably that which made me doubt the whole thing.
I was, unfortunately, Christian (as much as a little kid can be) in the first few years of my schooling for the simple reason that it got taught in school, and my experience is the best reason I can give for not teaching religion in school.
Due to Australian schools not having the First Amendment to protect them from religion, in NSW public schools the government has allowed a certain amount of time each week for members of the local religious community (always Christians) to conduct classes proselytising the students. Of course I believed when I started Kindergarten - how could I not? You went to school and learnt that 2+2 is 4, and that you write a k like so, and that Jesus died on the cross for you and healed lepers and blind guys all over the place. If you question one, why not the other?
Well, being of above average intelligence led to me questioning our scripture classes, (not to mention that God didn’t do anything to get me out of an undeserved detention when I asked him to), so rocketships and the failure of prayer ended things for me - the latter is still a convincing argument as far as I’m concerned.
From then on I never wished to go to scripture again, and my parents were fine with that, them not being religious either (I still don’t like the word atheist - it feels like I’m defining myself as something opposite to other people, when really I feel more like I just don’t bother with religion). Going to religion classes and the few times I attended church services just made me feel intellectually and morally dishonest.
Unfortunately, while I didn’t go to religion class anymore, everyone else did. And this is not because it was a Christian school - Australia is a rather secular place - but kids given the option between having to specifically get their parents permission to opt out from religion and stand out from everyone else or just attend a weekly scripture class, it’s fairly obvious they’ll take the easy route. So I was made to be very obviously seperate from everyone else, and it did make me an excellent target for the few serious Christians in the school, who then felt it was their duty to convert me and wouldn’t let up on it.
Eventually I got to high school where religious classes weren’t run during school time and I wasn’t conspiciously different. I got to know active Christians as people who actually did believe in their faith other than by default and didn’t go around pushing it on other people, and this helped me get over the dislike that I’d developed for the religious.
(Sorry to go on a bit, but to explain why I’m an atheist requires explanation of how I came to be one, I feel, and I needed to provide context to explain that.)
Today, I can’t really point to any one thing that leads to me deciding that god doesn’t exist. Having been an athesist for so long and paradoxically living in a secular society while still having to conspiciously define myself as an atheism means that it’s just a part of life for me. Everything I see suggests that there can be no god, from the lack of proof of its existence to features of religion that suggest it is man-made.
Religion bears all the hallmarks of something created by society rather than something that created society. Of course religion is artificial - you ever noticed how the people that decide that the Christian God is the one for them tend to live in overwhelmingly Christian countries, while the same can be said for the followers of other faiths? Or how its place in society changes according to the knowledge of the society? Once religion explained thunder, then it explained who was a witch and who wasn’t, and now it explains whether two men should be having sex or not. If religion was independent of society, if it was the word of God, then what was once true should still hold now. We should still be calling people witches and saying that thunder is made by God. We shouldn’t have explained some parts of the Bible as myth and story because it turned out that they couldn’t be true.
If the Bible is the word of God, I don’t see how anyone could possibly pick and choose. How can one say “God meant this, but not this,” without tacitly acknowledging that religion and God are man-made concepts?
And of course, religion always seems so restrictive - you’ve got to do this, not do that, go here then, eat that, wear this - why would any god care? And the rules are always so absolute. They don’t allow for personal circumstances or variations. I think it’s a better idea to listen to myself rather than get my life philosophy out of some two thousand year old book (that tells me I can keep slaves under certain conditions).
I also entirely reject the idea that it cannot ever be proven that god doesn’t exist. I understand that god and religion are two different things, but they are closely related, and throughout human history we have been proving that God doesn’t exist. Once, the sun was a god driving a chariot across the sky. We proved that wrong, and God became redefined as something else. Darwin’s theory of Evolution came along and showed that certain elements of God weren’t true, so people redefined their beliefs to accomodate this. When it’s proven that certain aspects of God are wrong, God is redefined to fit in with those aspects. God is our creation, and it’s no wonder that the God people believe in today isn’t the same as the one of the Old Testament or even the New Testament. If we were to ever gain the knowledge to prove people’s current conception of God wrong, then people would simply redefine God to allow that he could still exist.
Well, there you are. It’s bits of “why I don’t believe,” although it doesn’t touch on the most important aspect, which is that there is no reason to believe, anymore than I should believe that a being is producing the voices in the head of a schizophrenic.
It’s by no means meant to be a proof, and it’s a bit rambling due to it being late at night, but I hope it gives you some insight.