While I will not defend atheism, it is a bit naive to think that moral rules can only be known by divine revelation and further, that atheists are by definition, moral relativists. That is simply not true. One can observe human behavior and derive a perfectly sound set of rules and values to govern social groups.
For example: Thou Shall Not Kill is pretty obvious to most people. If you want to get along in a group it is simply wise to make an agreement not to kill each other and punish those who violate the rule.
BTW, I would not give you as strong an argument if you had said that most moral relativists are atheists (at least at heart). That is much closer to the truth. The opposite is simply not true. There are many moral, upstanding, responsible, Christian Law abiding atheists.
Ethics are relative,......Morals ARE Absolute!
Its "Thou Shalt Not Murder"