First and foremost, know that you are far from alone. I have spoken with a great many atheists who feel exactly like you do. These are human feelings and nothing to be ashamed of. But because they are human feelings, atheists are not the only ones who have them. For example, there are a great many religious people who think that atheists are "stupid" to believe as we do and can't understand how we can not believe in a god when they think that their god's existence is so obvious. This is why it is so important for us to learn to understand each other even when we don't agree on many subjects. This is why you have taken an important step by trying to grapple with this issue.

There's nothing wrong with not getting into religious debate. Some people like to discuss religion (or politics or social problems or history or conspiracies or fringe science, etc.) and some people don't. This is just a matter of personal taste. But remember that when you say you are irritated by religious debate because religious people are thinking without reason, remember that some of them may feel the same way about you. Also, as you read through the discussions on this site, I think you'll see that there are a fair number of religious people out there who are quite intelligent and reasonable. That they don't agree with atheism may be a matter of difference of opinion or of honest ignorance -- neither of which is a sign of stupidity. (As an aside, I completely admit that there are many religious people who are willfully ignorant, pig headed, or just plain unthinking, but I've also met many atheists who fall into these camps.)

There's also nothing wrong with thinking that religion should be kept out of science, law, and schools. Nothing that isn't science should have a place in science. There are too many fundamental disagreements about morality from a religious perspective to allow religion to dictate law. And religious education is best left to parents and religious educational establishments. None of these beliefs make you intolerant of religion, since none of them are anti-religion.

Personally, when I learn someone is religious, I don't automatically consider them stupid, perhaps because I have met so many religious people who are not stupid at all. I admit that sometimes I am disappointed when I learn someone is religious, but that is an emotional (as opposed to rational) reaction, and likely stems from the wish we all have that those we like be much like ourselves. On the other side of the coin, I have had the experience of telling someone I am an atheist and having them respond, "Oh, and you seemed like such a nice person." So it appears that atheists aren't the only ones who can have difficulty in this area.

I think you hit the nail on the head when you suggest that a way to get around this problem is to try and understand why people are religious. I'm going to give you my take on this, and warn you (and those who will read this online) that this opinion is completely from an atheistic perspective and therefore of necessity biased.

So, here we go -- why are some perfectly reasonable people religious. I can think of a number of possible reasons:

1) They were raised religious. This may not seem like much of an explanation, but for many it is sufficient. Lots of people who are raised with religion have theism so firmly ingrained in themselves that there's no way it's going anywhere. Even if they do come to the conclusion that there is no necessity for a deity, they will never shake the feeling that there is a deity, and that's what makes the difference.

2) They have an emotional need for religion. Some people cannot be content unless they "know" that there is something after death, or that there is a great master plan, or that there is a divine purpose to their life, or that they are loved and will be taken care of, or that there is a reason things happen, or that the universe can be easily explained, or that there will ultimately be justice in the world, or that miracles happen. Others have turned to religion for comfort while struggling with personal tragedy. These people may gain more from religion than they might lose by rationally examining their beliefs.

3) They believe that a deity is the best explanation for the universe. I think that science will be able to fully explain the origin of the universe without resorting to invoking the supernatural. I also think that scientific theories should be exhausted before we start assuming gods. Some people do not agree. They think that there are things beyond the reach of science, or that science has already proved that a deity is necessary. This is just a disagreement on a scientific subject.

So why shouldn't you look down on these people? Because you can't blame someone for how they were raised, for trying to fill their emotional needs as best they can, or for disagreeing with you about the necessity for the supernatural. Can you be upset if they say that anyone who was raised with different beliefs is a bad person? Sure. Can you be upset if they say that anyone who can deal with no life after death deserves to burn for all eternity. Yep. Can you be upset if they try and make the supernatural part of science education? Definitely. But these things have nothing to do with why someone believes in a deity, only with how they let their belief influence their actions.

Before we go any further, I should point out that someone could be an atheist for any of these reasons as well. You could have been raised an atheist. You might find comfort in a world without the uninvestigatable supernatural. You might believe that science proves atheism. I would say that none of these are rational reasons to be an atheist, any more than they are rational reasons to be a theist. But I don't look down on someone just because they feel this way. (By the way, I have seen people turn from atheist to theist based on both reasons #2 and #3 above.)

You mention not being completely convinced that religious people believe their god exists. This is not surprising. I have spoken with religious people who do not believe that the ancient Greeks really believed that their gods existed. I think this is because it is difficult -- and maybe a little emotionally threatening -- to imagine that someone might be as intelligent as you but have a radically different world view. Think of it this way: let's say you are a guy and your best friend (also a guy) is in love with a woman so incredibly annoying that you can barely be in the same room with her, and so unappealing that you could never find her physically attractive. Is it possible that your friend is correct when he says that he loves her, enjoys being with her, and wants to start a family with her? Sure it's possible. You can't for the life of you mentally put yourself in his shoes, but that doesn't mean that he is necessarily insincere.

I hasten to point out here that no all religion-based behavior need be tolerated. You don't have to put up with a religious person's abuse of your beliefs. You should certainly defend yourself when people try and use religion to discriminate against, abuse, or put down others. And you don't have to respect someone -- whether theist or atheist -- who thinks that a "discussion" involves yelling, calling names, and disparaging those who disagree with them.

Hopefully I've helped you see that religious people shouldn't be looked down on just because they are religious. Where do you go from here? I have three suggestions.

First, it doesn't sound like you enjoy debating religion, so don't go looking for debate. But if someone starts a discussion with you, don't automatically turn away from it, either. If someone says they have a scientific proof for God's existence, instead of debating their science, ask them if this proof is their reason for believing in God, and if they'll become atheists if you disprove it. If they say no (and they almost definitely will), ask them why they are religious. You'll learn more about their beliefs, and since an honest answer will likely have more to do with emotion or upbringing than science, there will be no reason for you to think them foolish. At worst, you will disagree with them, and there's no shame in that.

Second, if someone asks you why you are an atheist, tell them. Many religious people think atheists all "reject" or "hate" religion, as opposed to just having nothing to do with religion. Tell them that (assuming this is true) you have carefully examined your beliefs and think that the atheistic view is the most sensible. If they want details, give them details -- but stay out of the realm of religion as much as possible. If asked, "Why don't you believe in God?" answer that you don't disbelieve in God, you just don't believe in deities because you don't think they're scientifically necessary. From there, discuss the scientific basis for your statement, not the qualities of a being you don't believe exists in the first place.

Third, be a good atheist. Don't hide your beliefs, but don't annoy people with them or act like they're superior, either. Be happy. Enjoy life. Be good to others, charitable, and honest. Show how wrong those who believe that atheists can't be moral or happy are. If you notice that someone has said something untrue on a religious subject, politely encourage them to check their facts or examine their beliefs. If you notice someone behaving contrary to their stated religion, you might (if appropriate) ask them how their behavior fits with their beliefs. By being a good example and encouraging people to examine their own beliefs, you will not only make the world a better place for atheists, you might even make some religious people comfortable enough to critically examine their own beliefs, and for many, that is the road to finding those beliefs unnecessary.

I hope that this great flood of words is helpful to you. Please let me know if I neglected to answer one of your questions sufficiently or if there is anything else I can do.