According to statistics the vast majority of people in the world believe in a god of some sort, and amongst those the majority believe in the Judeo-Christian God (see note 1) so is it not just sensible to accept that god exists? Well, no. Certainly not to a skeptic. False beliefs amongst the majority are a common feature in history. The vast majority of people thought the Earth was at the center of the Universe at one time, they thought the Earth was only a few thousand years old, further back they thought the Earth was flat (see note 2). We now know beyond any reasonable doubt that all these beliefs are wrong. So clearly, belief by the majority doesn't mean something is true.
So are there any other reasons to believe a god or gods exist? There are several religious books in existence (including the Bible) which describe the existence of god. Many of these also have some historical accuracy. Unfortunately they also have a lot of inaccuracy. The Bible is just full of errors (see note 3), for example, so it doesn't support the idea of the existence of god to a skeptic (or anyone else who is prepared to look at the facts objectively).
What about physical signs that god exists? Well there aren't any that aren't better explained using natural theories. The appearance of life on Earth could be due to the intervention of a supernatural influence, but the theory of evolution (see note 4) fits a lot better, and doesn't involve theorising the presence of an unprovable entity (god). The origin of the Universe itself also fits into this category. In fact, there is a reasonable scientific explanation for every phenomenon we have observed. At no time has the presence of a god been found to be necessary. That doesn't mean there can't be a god, it just means we haven't found any reason to think there is one.
If there is no physical evidence, what about miracles, and other phenomena which cannot be explained naturally? Well, here again there are no examples of miraculous or unexplained events which have been examined carefully and not explained in natural terms. Faith healing has been examined and was found not to work, for example (see note 5).
Some people gain meaning from their life through belief in a god. Others believe that without a morality imposed by a god we have no absolute standards for right or wrong. But if we don't know that the god who supposedly originated these ideas really exists, they cannot be trusted. Many organised religions make use of their followers to give them power and money. And religious moralities come from the people who created the religion. Often the basic morality has some merit, but blind belief in a system of morals doesn't allow change and improvement as the situation changes. For example, the morality of Christianity doesn't guide us appropriately on topics such as equality of the sexes or environmentalism, for example.
Finally, what about god as a purely abstract concept? Einstein referred to god on several occasions (see note 6), but he didn't mean a god in the conventional sense. In these situations I think its misleading to use the word "god". If a physicist means he thinks the laws of physics are mathematically beautiful he should say that. If an astronomer finds the awesome size and power of the Universe inspiring why not say so? Making pseudo-religious comments and vague references to god doesn't really do these subjects justice. The more positive aspects of religious beliefs have been spoilt many years ago by organised religion.
In summary, belief in a god just creates more problems than it solves. For example, some people think god must have started the Big Bang because how else could the Universe have just appeared from nowhere? But if god created the Universe then what created god? In this case the explanation has just got more complicated without adding any benefits.
I don't know of any religion with a god which has any real chance of being true, but god as an abstract concept isn't necessarily that bad, although there's no reason I know to believe a supernatural world exists. Therefore I give this a moderate score on the crap-ometer!
1. According the Adherents Web Site 33% of the world population is Christian, 21% Islam, 0.22% Jewish. Non-religious is the third biggest group at 16% and other religions make up the remainder (some beliefs, such as Buddhism might not be religions in the usual sense).
2. Many people believe that belief in a flat Earth only ended recently, but the fact that the Earth is spherical (roughly speaking, its actually closer to an oblate spheroid) has been accepted for many years. There is an interesting Wikipedia article on the subject at Wikipedia, Flat Earth.
3. There are so many web sites dedicated to pointing out the errors, inconsistencies, and evil messages in the Bible that I hardly know where to start, but this site, the Skeptics Annotated Bible is one of my favourites!
4. Evolution is one of the best supported theories in science. While many people, quite correctly, point out it is "just" a theory, it is one of the few theories in science where there is no viable scientific alternative. There is also no serious reason to doubt its basic accuracy. See my evolution section, URL below.
6. Einstein didn't believe in a god in the conventional sense. Here's a couple of quotes: He did say that "God does not play dice [with the universe]", but he used god in a theoretical sense and as a convenient concept to make a point. This one shows his real ideas better: "I believe in Spinoza's God who reveals himself in the orderly harmony of what exists, not in a God who concerns himself with fates and actions of human beings."
Sources of Further Information
There are many web sites with information on this subject. Below I have shown some which present the information for both sides of the argument.