Wow.. what a great question! I'm one of those people who questions/challenges everything and goes through all the hard questions to make sure i'm on the right path. When I was about 12 I experienced the climax of all the doubts about god I'd felt creeping up on me for some time - a crisis of faith. I didn't know what to beleive. My pastor, rather than try to convince me, said to always keep an open mind. I took the advice to heart and feel I'm a much stronger christian because of it.
For me, the 2 possibilities that make the most sense right now are Catholicism and athiesm. After much deliberation, I've decided to stick with Catholicism.
The athiest would have me beleive that the universe has always been here, has always been moving forward in time, since an infinite amount of time ago. That it will continue to do so forever, for no reason at all, and happened to produce a world like this, with intelligent beautiful creatures like us as well as birds and fish and flowers. This, to me, sounds quite absurd. All change has a cause, so change itself must be caused by something unchanging. Given the world we live in that seems to be the Christian God (see CS Lewis'
Mere Christianity)
I also have gut feelings about the matter of my own conciousness that I cant really describe but which, to me, prove the existence of something beyond my material body that defines who I am.
The athiest would have me beleive that one criminal and 12 friends of his decided to become poor homeless people and ultimately martyrs, oppressed by the great Roman government for their whole lives. Furthermore, these liars somehow created an underground religion that would spread to encompass billions (trillions?) of beleivers all over the world. Yeah right.
There are many other reasons, which I wont go into, which also affirm my faith. Mere Christianity by C.S. Lewis is definitely a great book, as someone mentioned already!
I was raised on Catholicism as opposed to Protestant Christianity, and I havent found a single doctrinal issue yet that puts me at odds with the Catholic faith. I
don't want to start a debate about it here but I felt compelled to mention it as long as I'm talking about why I beleive what I beleive. The protestant reformation sparked an exponentially increasing divergence in the Christian faith which I find unfortunate (though obviously some consider it necessary). The one good thing to come of all this denominationalism (IMO) is that it challenges us to look at the little details of what we beleive about God and the nature of Christianity, rather than generalizing about the beleif system. Just goes to show everything is part of God's divine plan for salvation!
This is a great topic.. keep posting, people!