Full Idea
On Kant's view it never follows that we ought to do what God commands, for we would have to know that we always ought to do what God commands, but that would need a standard of moral judgement independent of God's commands. God's commands are redundant.
Gist of Idea
We can only know we should obey God if we already have moral standards for judging God
Source
report of Immanuel Kant (Groundwork of the Metaphysic of Morals [1785]) by Alasdair MacIntyre - After Virtue: a Study in Moral Theory Ch.4
Book Reference
MacIntyre,Alasdair: 'After Virtue: a Study in Moral Theory' [Duckworth 1982], p.43
A Reaction
This strikes me as a very powerful argument, even an undeniable one. How could you accept any authority if you didn't have some standards for accepting it, even if the standard was just to be awestruck by someone's charisma and will-power?