God and the Anthropological Question

Michael S. asks whether a Christian anthropology provides a truer/richer account of our obligations to each other than does a secular dependency care theory founded on reciprocity.  If our objective is to articulate a moral framework, does it matter whether we are created for a purpose or whether we simply exist by happenstance and assign ourselves a purpose?  Either way, someone can defy that purpose and reject the claimed moral obligation.  The consequences will be more severe if I'm rejecting God's purpose -- whether because of eternal damnation or my failure to realize my own full flourishing -- but does it end the moral inquiry simply because God said it (through the act of creation) rather than John Rawls saying it?  God told Abraham to kill his son -- that command is begging for moral criticism, isn't it?  As H.L.A. Hart wrote:

The moral monster who thinks there is nothing morally wrong in torturing a child except that God has forbidden it, has a parallel in the moralist who will not treat the fact that the child will suffer agony as in itself a moral reason enough.

So does Christianity's advantage over secular theories of justice stem from its recognition of the anthropological implications of our status as created beings -- i.e., that the facts of our natural existence (gender difference, parent-child relationships, etc.) are not accidental, but reflect our creator's intentions?  If so, does the advantage disappear once someone decides to contest the morality of those intentions?