A very popular contemporary debate between theists and atheists concerns morality. The theist argues that human morality must rely upon some transcendent and therefore divine source in order to carry any weight; anything less results in a purely subjective ethics that is too relative to ascribe objective goodness and badness to man's actions. Atheists argue that morality relies upon evolutionary biology and sociology which require no such transcendent or religious authority.
Part of the reason why this debate is so popular is because we have forgotten our philosophical ancestry, which had its culmination in the medieval ages but began with the ancient Greeks. The existence of God is not some hypothesis employed in order to explain certain phenomena, it is a conclusion that is reached after the long, arduous, and sometimes tedious exploration of reality as a whole. Aristotle arrives at God in the twelfth book of the Metaphysics, and it is no simple task to follow him through from beginning to end.
But if you do follow the ancient way to God, you will discover that God is an absolute necessity to the existence of the world. Once you understand the distinction between act and potency, form and matter, essence and existence, etc., you understand that all things that are not God need God in order to exist. It is a question of science, not faith. God is the explanation for reality when considered at its most general level. He is a type of scientific explanation. It is not as if we can ponder what things would be like without Him, His existence is at every moment doing explanatory work.
So the interesting question is not whether we can be moral without God, the essential question is and always will be "How can we be without God?" Act an potency are real, essence and existence are real, and form and matter are real. All of the metaphysical distinctions are real. It does no good to deny God's existence as if His being is inconsequential. If He does not exist, you have left an infinitely wide explanatory gap that simply cannot be filled by any empirical science.