If there is no God, there is no morality either. Only a transcendent, personal being like God can serve as the ontological foundation for transcendent moral truths and moral duties. Cultural norms and mores may still exist without God, but not moral truths. Without God to provide the ontological grounding for objective moral values, what we refer to as “morality” is nothing more than expressions of our subjective preferences or human pragmatism. To say “murder is wrong” is no different than saying “chocolate ice-cream is gross” or “you shouldn’t drive on the left side of the road.” Moral obligations fall by the wayside, for in the name of what ought anybody submit to cultural preferences or pragmatic mores?
To believe morals exist but God does not is like believing books exist but authors do not. There wouldn’t be any books in the absence of authors, and there wouldn’t be any moral truths in the absence of a transcendent, personal, holy God to ground those moral truths in reality. Put another way, to believe moral truths exist in the absence of a transcendent source like God is like believing books exist in the absence of authors. And to believe that we are obligated to behave in certain ways in the absence of a moral law maker and judge is tantamount to thinking one is obligated to obey the laws in a nation without legislators.