According to Kantian moral religion, because there is a duty to promote the highest good, we are warranted in believing in God and immortality. However, this article shows that the duty to promote the highest good is incoherent, and that popular conceptualizations of the highest good cannot avoid this incoherence. After arguing, additionally, against attempts to ground Kantian moral religion on the highest good in some role other than the object of duty, it is shown that Kant seems to have (...) been aware of the original problem, suggesting an explanation for why he abandoned the highest good around 1800. (shrink)