(1) expresses a broadly logical equivalence and equivalence is symmetrical: if p is equivalent to q, then q is equivalent to p. But metaphysical grounding is asymmetrical: if M metaphysically grounds N, then it is not the case that N metaphysically grounds M. For example, if fact F is the truth-maker of sentence s, then it is not the case that s is the truth-maker of F. Truth-making is a type of metaphysical grounding: it is not a causal relation and its is not a logical relation (where a logical relation is one that relates propositions, examples of logical relations being consistency, inconsistency, entailment, and logical independence.)
(1) leaves wide open whether God is the source of the obligatoriness of moral obligations, or whether such obligations are obligatory independently of divine commands. Thus the truth of (1) does not entail an answer to (Q).
The 'because' in (Q) cannot be taken in a causal sense if causation is understood as a relation that connects physical events, states, or changes with other physical events, states, or changes. Nor can the 'because' be taken in a logical sense. Logical relations connect propositions, and a divine command is not a proposition. Nor is the obligatoriness of the content of a command a proposition.
So I say this: if the content of a command is morally obligatory because God issued the command, then the issuing of the command is the metaphysical ground of the the moral obligatoriness of the content of the command. If, on the other hand, the content of the command is morally obligatory independently of the issuing of the divine command, then the moral obligatoriness of the command is the metaphysical ground of the correctness of the divine command.
Either way, there is a relation of metaphysical grounding.
My argument in summary:
1. (Q) is an intelligible question.
2. (Q) is not a question about a causal relation.
3. (Q) is not a question about a logical relation.
4. There is no other ordinary (nonmetaphysical) candidate relation such as a temporal relation or an epistemic relation for (Q) to be about.
5. (Q) is an intelligible question if and only if 'because' in (Q) expresses metaphysical grounding.
6. 'Because' in (Q) expresses metaphysical grounding.
7. There is a relation of metaphysical grounding.
OK, London Ed, which premise will you reject and why?