It can be very reassuring.... seriously. My family are bible thumpers to the nth degree.
I once served with the same vigor as they. I chose "serve" intentionally. When you are a believer, its very easy to focus on the parts of the bible that speak to you (not literally).
You also see many of your church members and family members develop very admirable traits that derive from their adherence to what they believe god wants out of them. Charitableness, empathy, cleanliness in attitude and speech, lack of debt and accountability in finances, etc. (Now, we all know there are plenty of exceptions - but I am writing of my experiences and the common man, here... not the Haggards and Bakers of the world.)
Couple those favorable attributes with the "worldliness" of the non-Christian populace (all the perceived crime, hatred, etc that you see and associate with the ungodly) and you have a huge support structure for your belief that god makes you a better person.
If one of your fellow Christian mates happens to falter (e.g. backslide), then it isn't because of god but because this person lost something... be it faith, be it discipline and seeking god first, be it to temptation, etc. This then propels you even more to focus on god and his teachings (i.e. the bible).
Second, as a theist, it is easier to attribute good things happening by divine influence. Get a lucky break here, a free pass there, etc... and to you it isn't a lucky break or a free pass - they are blessings. And, if you try to associate something with god in your daily life - and you hang out with people that associate god in their daily lives, it becomes all encompassing and self-validating.
Third, during struggles, it can be reassuring because you believe that god is on your side. It may be a struggle, it may be a trial - but it will come out to god's will and for the betterment of mankind by god's devices (mysterious ways). Plus, you always have god to talk to... you always have god to cry to.. etc. Couple that with the other Christians listed in the first paragraph. If they have taken on the loving, nurturing and supporting role - then they come to your aid. Out of the love of god... so then it becomes all encompassing and self-validating.
Further, the more you read the bible - the more time you devote to it and to praying and to digesting passages here and morals there... then it becomes alive to you. It becomes rock solid and only the more strenuous circumstance or clear cut arguments will register against god in your mind. (I am talking about the average, sincere and devoted fundamentalist Christian here - not the church every now and then, half-ass Christian.)
In all truth, your mind can make life (reality). It can shape it, frame it and then keep that model alive. It becomes a part of what and who you are and how you define yourself. It becomes a part of what and who the world is and how it interacts with you, and vice versa. It is all encompassing. There is much reassurance, security and benefit to be found in it - as least, that is how you see it. And the love you begin to develop for it - and the perceived love you begin to feel and attribute from god become real to you. In fact, its isn't that it becomes real but that is has always been and you now see it, accept it and bury yourself in it. It is more than a "I believe in Jesus" - it is and it shapes who you become and what your world is... and that is perhaps the paramount reason that it continues to propagate and to draw the fanaticism that it does.
Written by El Guapo of AvC