I recently read an interesting and fun book. It was The Probability of God by Stephen Unwin, and it attempted to show how one could use various facts about our world to compute the probability that God exists.
Unwin works as a risk assessor for large US companies, and uses the mathematics of Bayesian probability in his work, so he thought he would try to use the same ideas to examine God. He considers factors (or "evidentiary areas") that suggest God might be more likely to exist, such as how we know what's right and wrong, miracles and religious experiences, and other areas that might suggest God is less likely to exist, such as the problems of natural and human evil in the world.
Unwin assesses the evidence and ends up with a probability of God 's existence of 67%.
Of course we shouldn't take it too seriously. The probabilities have no factual basis, and are simply his subjective assessment. But I found the exercise interesting, because it allows everyone to use their own estimates of probabilities and come up with their own answers. And allows each of us to test our belief against something that at least tries to be even-handed.
In fact I found it so interesting that I wrote a program that allows the test, upgraded with a few more questions, to be taken on-line by anyone who wishes. Testing the program indicates you can get any answer, from 0 to 100%, so it doesn't seem to be biased. So if you're interested, you can make your estimate of the probability of God on my website. I'd be interested to hear what you conclude.
For the record, I calculated the probability of God to be 98%, which I guess tells you something about my answers.