Many people believe that morality and religion are fundamentally linked. They may see this as an argument for God's existence (if atheism's true, there can be no moral facts; moral facts exist; therefore God does too), but in this essay I shall evaluate it simply as a factual claim. If you like, you can see me as trying to undermine the first premise of the argument I just gave, claiming that morality can - and in fact must - have a purely secular foundation.
One reason why some theists think that morality presupposes God's existence is that their beliefs seems to give their lives clear purpose, making them wonder what purpose atheists can supply for themselves (Trueblood, 1957). According to Christianity, we have been created by God in His image, so as to fulfil the purpose He intended us for. The Catholic Church developed this basic idea into the concept of natural law: we can see how we should behave by working out what our purpose is; often, it will be manifest in the way we have been designed. The Pope's condemnation of homosexuality is an example of this theory at work: the natural result of sex is conception, and any sexual acts not open to this possibility involve an immoral rejection of the purpose for which God intended them. Needless to say, this approach to ethics is controversial; it might be felt to be a poor imitation of morality. Regardless, few now think it will do as a foundation for it: it begs too many questions, and fails to provide a justification for many of our most basic moral principles (Gary, 1999).
A different explanation of the purported link between God and morality is the idea that we have an obligation to obey His commands because He is our omnipotent creator, to whom we owe our existence. The obvious question is: "Why do we have this obligation?" Might doesn't make right, and though people once believed that children owed obedience to their parents simply because they depended on them, this view has not stood up to reflection. It is rather like the justification given for feudalism: masters protected their serfs, and it was only fair that they receive a share of the crops in return for this. The flaw is of course that the serfs never agreed to ...