I've been very struck by much of what I've read in this thread. I have a slightly different take on the question, which I'd like to share here.
I'm a Christian. I believe that Jesus was God; that there is only one god, and that god is all-powerful and absolutely fair (or "just" or "good"). My reason has to do with my conviction of the unfairness of life.
I believe that, like most or all other people, I was born with certain faculties or senses -- specifically, a sense of reason or logic, and perceptory senses -- which tell me that life is unfair. Life is incredibly unfair for lots of people, and my own life, in spite of all my privileged circumstances, has still been unfair in many ways. Actually I think that's true for everyone, regardless of their circumstances.
My conviction of life's unfairness is not merely an intellectual belief, but something I feel deeply. It's based on a lifetime of personal experience. It's something that makes me very, very upset, and I believe it's supposed to -- the unfairness of the suffering in the world around us ought to inspire not just sympathy, but anger, and even despair.
My senses of reason and my perceptory senses tell me that in a world of such unfairness, there cannot be an all-powerful and just god, because no such god would allow such unfairness. (In my opinion, that's the answer to the "why not believe in god?" question that others have mentioned here, at least with regard to the God of Christianity.)
I believe Christianity tells me that ultimately, I cannot trust my in-born faculties and senses; I cannot trust my own logic or reason, or the evidence of my senses, which tell me again and again that life is unfair; in essence, I cannot trust myself, because I am defective, broken. (In my opinion, many of the reasons for belief presented in this thread are based on logic or the evidence of the senses, which is why I personally disagree with them, even though I strongly sympathize with the explanations others have presented here. I believe that trusting in logic or the evidence of the senses will ultimately lead me away from the truth, not towards it.)
I choose to believe Christianity -- to believe in Jesus, in the Christian God -- because to do otherwise would be to trust in my own senses and faculties, which tell me life is horribly unfair for most, if not all of us, and there cannot be a just god in a world that's so unfair.