On Kant's view it can never follow from the fact that God commands is to do such-and-such that we ought to do such-and-such. In order for us to reach such a conclusion we would also have to know that we always ought to do what God commands. But this last we could not know unless we ourselves possessed a standard of moral judgment independent of God's commandments by means of which we could judge God's deeds and words and so find the latter morally worthy of obedience. But clearly if we possess such a standard, the commandments of God will be redundant.
p. 45. - After Virtue (1981)