The moral argument, stated at its simplest, asserts that moral facts, or imperatives, exist, and that the best way of explaining these is in terms of a supernatural moral Being (i.e. God).
In asserting that moral facts exist, the Christian apologist is on common ground with the majority of philosophers, including those who are not theists.
Fewer, however would accept that the existence of moral facts necessarily entails the existence of God.
Some would try to explain the moral impulse in terms of social pressure and cultural conditioning. But some of the moralists we most admire have been precisely those who have resisted prevailing attitudes and mores.
Evolutionists might claim that altruism is necessary for the survival of the species, and therefore is attributable to evolutionary processes of natural selection. In response to this particular objection, we might observe that the exercise of conscience often does not benefit the species in terms of survival, and it is therefore unlikely that conscience developed by means of natural selection.
As formulated by Kant, the moral argument had a distinctly eschatological flavour. The moral law commands us to seek the highest good, but the only way we can make sense of that is to postulate a supernatural moral Being who will reward moral endeavour in another world.
Peter Berger has offered a negative version of the moral argument which he calls ‘the argument from damnation’. Some people, he says, are so transcendently immoral as to demand a condemnation of supernatural proportions. In our very condemnation of extreme wickedness, we point to a transcendent realm of moral absolutes.