I think Grace is talking about irrational concept of "grounding" your morality; that is defending it against the infinite regress of "why's". He thinks that God can somehow ground morality and that atheists cannot do this since they do not believe in God. What I think he misses is that every moral system is based on axioms, such as "Do not treat others as mere means" or "Do unto others and you would have them do unto you", and that his system is no exception. I would say he acts under the axiom, "Whatever the Father does is good" or something to that effect.
I don't see how you can defend how one
should act (the concept of a moral action) without following with the statement "if you abide by the X principle(s).
I may be completely off, so I'm sorry for interjecting if that is the case. I think morality is a sort of controversial atheists and theists alike. We are all on the same boat