Morality is derived from the desire to want to give one's offspring the maximum chance of survival. Humans, like animals, have the greatest compulsion to protect and act altruistically to the immediate offspring and family, and lesser compulsion to social circles, to society and to the species as a whole. You generally don't see animals within one species or one group slaughtering each (well no more so than humans!) Why is that? Is that not morality amongst animals?
It makes perfect evolutionary sense, and has an underlining evolutionary compulsion. Certain brain injuires can reduce or destroy and individuals ability to make moral decisions, so we even have a physiological basis.
It is then varied greatly by environment and culture (and individual aberration such as injury or mental illness).
Christians who propose this argument only have to look at the history of the Christian church, or indeed look throughout the mulititude of Christian denominations for an extremely varied manifestation of morality.
There is a whole stack of rebuttals of the argument from morality here:
Theism Moral » Internet Infidels
And to gut all your other arguments, look here:
Theism Arguments » Internet Infidels
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