Morality comes from beliefs. Beliefs are a form of knowledge. They are extra-rational knowledge.

Atheists have beliefs, such as a belief in the equal moral worth of all people. Like all beliefs, to accept that belief is an extra-rational act. Like all beliefs, accepting it makes it valid: beliefs attain validity through their (extra-rational) acceptance.

"Objective" knowledge gets its validity from observation within material existence, using the rational faculty (which, I believe, God gave us). Such knowledge can provide an ethic of justice: mutual respect (in effecting choices). It follows from the observation that human beings--and societies--have no choice but to effect choices (i.e., choose among perceived alternatives and take action to bring that choice to fruition).