It's been a long time since I last read Ayer's comments on morality and religion, and so I don't have much to say in response to that aspect of your question. However, it seems clear that the force of morality cannot depend on the coercive sort of authority you describe, whether that authority is wielded by God or the state. God's threats of punishment carry no more moral weight than anybody else's. And so if murder is wrong because God forbids it, God's authority must extend beyond his (admittedly impressive) power to cast us into hell for all eternity.

The sort of authority that morality is supposed to have is not the coercive authority of reward and punishment, but rather what philosophers often call normative authority. (Precisely what this sort of authority amounts to remains the subject of much philosophical disagreement.) As I mentioned in my response to a different question, many philosophers think that normative authority must ultimately be independent of God's will and commands. Thus it seems that any compelling account of the foundations of morality is going to have a humanistic rather than religious basis. One of the great challenges of moral philosophy is to explain how such foundations are possible. How can any commandment or law--of divine or human origin--have genuine normative authority?

A nice summary and explanation of various attempts to answer this question can be found in Stephen Darwall's excellent introductory book, Philosophical Ethics.