My question though is "where does their/our sense of morality come from?" Where did this code come from that you say even atheists can abide by?
"Whose" morality is it that atheists are abiding by? They may abide by it but they didn't invent it...or, if they did, what is the basis for it?
As CS Lewis said, humans have (some do, at least) this powerful inner sense of right and wrong. Did we learn that from our parents, our society? If so, where did our parents and their parents learn it from?
Animals don't have it. Peoples whose civilizations are not based on or influenced by Judeo/Christianity don't have it (our sense of morality that is).
Atheists may be moral as we define morality, but perhaps they are just following the rules of our society...a society founded on the principles of Judeo/Christianity.
And just what principle of Judeo/Christianity lead to the abolition of slavery?