I can.
Sorry, you can not.
Morality comes from God, it is NOT defined by the individual - just the way it is.
Ok, I get that an atheist wants to be relevant.
However, we can be in several states: moral, ethical or legal.
Morality is not defined by the individual, it IS founded in God - a place an atheist can not be.
Ethics which is founded in morality, also leaves the atheist in the wind.
So that leaves legal,,,,,the best an atheist can be is legal and yes even that is iffy.
I have said it before....that is the best you can be.