Why do some people believe in the religion that they believe in and not another religion?
Why do some people consider the existence of God to be self-evident while others think that belief in God has no basis?
Why do some people believe that the Bible is the word of God while others believe it is only the words of men?
I think the reason people believe in a religion or in God can be objective as well as subjective.
Even though there are objective facts associated with all religions, people are going to interpret those facts subjectively, which is why we don't all adhere to the same religion.
Regarding subjectivity, why do some people love cats and others love dogs, why do some people like rainy weather and others like sunny weather, and why do some people feel a need for a romantic relationship while others do not care about that at all?
The list goes on. We all have different desires and preferences so why would that not apply to the religion we choose to believe in?
I think the initial basis for religion has to do with exposure in the early years of our lives when we are very vulnerable to new ideas and don't really have a lot of experience of our own. Hardly any children question their parents, so if one is told that God exist and it is this particular one, you have no reason to assume that your parents are lying or most likely wrong about it. And when that is the case, all their ideas and views are imprinted on the child as they grow up.
But this only goes so far I think, because at some point everyone will start asking questions as they get experiences of their own and other external influences and views and therefore they start becoming more independent. The parents are no longer these beacons of knowledge that can't be wrong.
At this point, things start to become a lot more complicated and it seems to now depend on several things. But first and foremost the individuals themselves. How do you deal with new knowledge and contradictory views to your own?
Some will investigate trying to figure out whether these contradictory views make sense or not? While others will simply dismiss them as being wrong. Which I think is closely related to humans in general not liking to be wrong and also this is closely related to comfort I think. We grew up in a world where things work in a certain way, God is there and he is watching, taking care of us etc. Now someone comes and says that he doesn't exist!! that pretty much shatters the comfort we are used to if you were to believe that. All of a sudden, the world you know is not the same then, not only for yourself but also it could potentially expose those you love and trust as being wrong. And also it could make you feel like you are outside the group, if everyone else around you believes in God and you suddenly don't, what about all the traditions etc.? How will they react to it etc? These for most people are extremely uncomfortable to deal with, especially also because in many religions you are not looked well upon for denying God and might even risk getting excluded or even punished severely for it.
In this case, you can either pretend and live a lie or you might lose your whole foundation. Again this depends on the religion you follow and the country you live in obviously. That is more of the personal side of the issue.
The second issue is how you as an individual approach these things and to me this is probably the most interesting part because this is about honesty and figuring out how you as a person want to deal with things that contradict your beliefs, not only religious but in general.
And I think this is where the biggest difference is between people, at least in my opinion I think atheists in general are more open to new ideas than a majority of religious people are, due to us not having to maintain or defend a religious view. Even within religion, not only do you have to "defend" against the atheist view you also have to defend against other religious views.
Whereas as an atheist, you question all the religious views doesn't matter which they are and you are not as such "fighting" against religion as I think a lot of religious people might think. Rather I would say that we are defending the idea of critical thinking and scepticism against wishful thinking, and religion just happens to be a huge player in this field, based on a lot of unverifiable ideas and people claiming to know things for which they have no basis.
And finally, to follow up with the personal side, I think a lot of people feel uneasy with the thought that we/they just exist basically without any reason or higher purpose. They want an answer to this, why am I/we here? and atheism doesn't offer this answer. But a God if you accept it, does offer an explanation as long as you don't question its validity. If you ignore this question, then you can jump ahead and start asking questions that you as an individual might find more interesting, which is, why does God do what he does? Do I live up to God's expectations etc.? What is God trying to tell us? and I should treat others according to what God tells me.
As an atheist, we obviously would like to know why we are here, but have no issues not knowing, because we have already been through the whole process of accepting that this is probably not something we will ever know. And therefore it is more important to spend time helping each other and get the best out of things as we can. This is also why a majority of atheists identify themselves as humanists and value science because it is what we consider the most valuable tool to achieve a better world and therefore better conditions for humans and animals etc.