One reason which some theists, especially Christians, will offer for thinking that the existence of their god is important is that belief in a god is good for, or even necessary for, social order and moral behavior. For hundreds of years, Christian apologists have argued that without a belief in a god, basic social structures would disintegrate and people would no longer find reason to act morally.
It is a shame that so many Christians (and other theists) continue to employ this argument this because it's so bad. The first point which should be made is that it obviously isn't true that their god is required for good social order and moral behavior - most of the cultures in the world have gotten by just fine without their god.
Next is the question of whether or not belief in any god or higher power is required for morality and social stability. There are any number of objections which can be made here, but I will try and cover a few of basic ones. The most obvious thing to point out is that this is nothing but an assertion, and empirical evidence is clearly against it.
An examination of history makes it evident that believers in gods can be very violent, especially when it comes to other groups of believers who follow different gods. Atheists have also been violent - but they have also led very good and moral lives. Thus, there is no apparent correlation between belief in gods and being a good person. As Steven Weinberg noted in his article Designer Universe:
- With or without religion, good people can behave well and bad people can do evil; but for good people to do evil - that takes religion.
Another interesting fact to point out is that the claim doesn't actually require any god to really exist. If social stability and morality are only achieved with believing in a god, even a false god, then the theist is claiming that human societies require massive deceit in order to survive. Moreover, the theist is arguing that a society doesn't actually need their god, since any god will apparently do. I'm sure that there are some theists who will quickly agree with this and not be bothered, but they are rare.
A more fundamental objection, however, is the implicit portrayal of humanity which such a claim makes. The unspoken reason why humans need some god to be moral is that they are not capable of creating their own social rules and, hence, require an eternal rule-giver with accompanying eternal rewards and eternal punishments.
How can a theist possibly claim this when even chimpanzees and other primates are clearly capable of creating social rules? The theist is attempting to create ignorant children out of all of us. In their eyes, we are apparently incapable of running our own affairs; worse yet, only the promise of eternal reward and the threat of eternal punishment will keep us in line. Perhaps this is actually true of them, and that would be unfortunate. However, that is not true of any of the atheists I know.
« Questioning the Importance of God | Purpose & Meaning in Life »

