C.S. Lewis. I want this quote to go viral because everyone needs to see it and reflect on it. Dr William Lane Craig has been under attack for arguing this same argument. Again it's not that people are unable to do moral acts and deny God, they just have no way to prove that its moral or condemn others - without borrowing from God's "measuring rod". We take that rod and twist it and mutilate so that we can pretend to fit it it we can look down our noses at others, forgetting that we are just as crooked as those we criticize in many other areas.
"If 'good' or 'better' are terms deriving their sole meaning from the ideology of each people, then of course ideologies themselves cannot be better or worse than each other. Unless the measuring rod is independent of the things measured, we can do no measuring, For the same reason it is useless to compare the moral ideas of one age with those of another: progress and decadence are alike meaningless words."
- C.S. Lewis Christian Reflections, p. 73
C.S. Lewis on the Meaning of Morality - Apologetics 315