A moral relativist believes right and wrong are relative to individuals and cultures. Moral values are "subjective." Therefore moral values are not objective.
If there are no objective moral values, and moral values are only subjective, then moral values are not binding on persons. Moral values then become matters of personal taste.
Only if there exist objective moral values can such moral values be binding on persons. Only if there is a God can there exist objective moral values. Objective moral values do exist. Therefore, God exists. For more on this see William Lane Craig's article "The Indispensability of Theological Meta-Ethical Foundations for Morality."
Note: Nietzsche and others have affirmed that atheism implies no objective "good" and no objective "evil." One goes "beyond good and evil" and is left only with "power." I find Nietzsche's atheism convincing in the sense that, were I an atheist, his views on the senselessness of objectively binding moral values follows consistently from the premise of there being no God.
