It doesn't actually matter. I am aware that Christianity is not the only source of moral philosophy. Aristotle realized this before Christ was born - Metaphysics, Book V. And I am aware that many people practice codes of morality without believing in God. My point is that in the absence of external authority these codes are arbitrary and foundationless. You're certainly free to disagree.
However, most religious moral codes change over time, Christianity’s included, even if the Scripture doesn’t. Slavery is a good example; what is accepted and regulated by Scripture would be utterly rejected as wholly immoral by most Chritians today. So to claim that religious morality is some how rooted to fixed, eternal moorings compared to that of non-believers is wishful thinking IMHO.