First of all, I'm an agnostic and don't know if there is a God or not. However, I do think it possible to intelligently review the evidence and come to some reasonable conclusions, even if we can't have absolute certainty. It's a big topic, so I'll just discuss the main reason that many people believe in God. It often comes down to a hunch. People have a gut feeling that there must be a God. So for many people, their belief in God is based on the fact that many people believe in God. I'm not trying to be disrespectful or trying to trivialise this. Rather, I think it is an important argument that needs to be discussed seriously.
If there is a God, it seems likely that we would be created with a kind of inborn belief in him, so this could be taken as evidence of his existence. On the other hand, this evidence would lose much of it's force if the theory of evolution made the same prediction. It's hard to analyse the survival value of believing in God, but a good argument can be made that there is a survival benefit to attributing some kind of intelligent cause to things that have an unknown cause. So if someone hears a rustling of leaves and assumes that it is caused by a predator or enemy, he will have a survival advantage, even if he is wrong most of the time. If natural selection over a long period of time selects for this way of thinking, then it is easy to see how belief in God as an intelligent cause for unexplained events could develop.
It might seem hopeless to decide which scenario best explains the evidence if they both predict the same thing. However, there are some differences in the details. The common gut feeling that there is a God is very powerful, but it can be overcome and it also varies from person to person. This is consistent with evolutionary theory, but it is not quite consistent with what we might reasonably expect if we were created by a loving God. If there is a God, he could have made this gut feeling absolute so we wouldn't have to waste time debating a certainty. Or he could have given us clear supplemental evidence that he exists. Neither is the case.
If there is a God. I would expect that he would not only give convincing evidence of his existence, but would also design us with the ability to properly understand and visualise the nature of the universe. If our gut feeling in the existence of God was just a consequence of a natural tendency to misunderstand the workings of the universe, then I would consider that to be strong evidence for the nonexistence of God (at least of a loving and wise God).
The fact is that our natural tendency to attribute an intelligent cause to unexplained things offers survival benefits, but greatly hinders us in our efforts to understand ourselves and the universe. This fact is obscured somewhat by modern science. We have learned that at least most of the workings of the universe have natural explanations. In the past, it was common to assume these were the result of supernatural causes. Think of all the reports of demon possession in the Bible. Before people understood illnesses and even minor maladies, they often assumed these were the result of demon possession. This natural tendency is pervasive. For example, Kepler discovered some mathematical laws of planetary motion. They described what happened, but not how it happened, so many believed that angels moved the planets according to these laws. Newton eventually discovered that a mathematically precise description of gravity could account for these observed motions, so the unnecessary hypothesis of planet moving angels was abandoned. I view this natural human tendency to be more consistent with the forces of evolution then with the hypothesis of a wise and loving God. Also consider how poorly equipped we are to comprehend and visualise advanced scientific ideas like relativity, quantum physics, extra or curved dimensions, etc. These abilities wouldn't have provided any survival benefits (until recently, at least) so it's not surprising that we don't have them. If we were created, I would expect that we would be better prepared.
I know this is just a brief review of an important topic, but I would like to hear your thoughts.