This essay presents a moral argument for the rationality of theistic belief. If all I have to go on morally are my own moral intuitions and reasoning and those of others, I am rationally led to skepticism, both about the possibility of moral knowledge and about my moral effectiveness. This skepticism is extensive, amounting to moral despair. But such despair cannot be rational. It follows that the assumption of the argument must be false and I must be able to rely on more than my own human powers and those of others in attempting to live a moral life. The Christian God has such a function. Hence, if it is rational to attempt a moral life, it is rational to believe in the Christian God