The Moral Argument is an argument for the existence of God based on the initial premise that morality is absolute. It was first offered and expounded in greatest detail by Immanuel Kant, and runs roughly as follows:
Morality is absolute
Given that absolute morality exists, we must be able to aspire to achieve its standard
Given that humans can freely choose whether or not to be moral, there must be an ontological reason for them to do so
It can be observed that being moral makes humans happy
Therefore, most moral humans will be happier than non-moral humans
However, a better system is needed to ensure that all people who are moral will be rewarded
Therefore, the afterlife exists
Therefore, an impartial arbitrator is also needed to judge humans' obedience to morality
This is God
In its defense, the moral argument provides a solution to the Euthyphro Dilemma- God does not create morality, but rather enforces it. This solution is, however, unsatisfactory to many theists, including divine command theorists, who wish to postulate God as the source of morality.
However, the moral argument is often ridiculed for having as its basis six of the most contentious issues in philosophy- the nature of morality, the fundamental nature of humans, what makes humans happy, free will, afterlife, and God. Indeed, it can be shown that the successful challenge of any one of these six items renders the entire argument unsound and/or flawed, as every concept relies on the successful demonstration of the others to hold true.
Furthermore, the growing acceptance of the diversity thesis and the dependency thesis has lead many people, even theists, to question the previously assumed universal nature of morality, destroying the first premise on which the argument is founded. Furthermore, recent studies from the fields of biology and sociology have suggested that common moral laws, such as a prohibition against killing members of one's own social group, have a distinct evolutionary advantage in increasing the inclusive fitness of the group, removing God and the afterlife, and again destroying the argument. These numerous flaws have lead the vast majority of theists to seek alternative arguments for God's existence.