I believe that in order for objective moral values to exist there must be a God who sets the intial standards by which each of us are guided to follow by. A set of values that transcends us so that we all may know what it is to be good or bad in our actions. Athesit can not prove that an action is good or bad because without a divine law giver morality simply becomes a set of opinions, each individual may have their own beliefs about what is truly right or wrong.
Take for example without a God, to say the holocaust was objectively evil and morally wrong even though the Nazi's who carried it out thought that it was good, would be to say your point of view is right while theirs were wrong is really a matter of opinion. And even if the Nazi's would have won the war and succeeded in taking over the world while convincing everyone they were right would still be an opinion without a divine certainty of what is objectively right or wrong.
While I may think to start a war that kills millions as objectively right, you may disagree. Then who is truly right in this instance ? And why ? It then comes down to Subjective rather than Objective because each to his own without a moral standard. Another example, take the death penalty for instance. If I were sentenced to death for a crime, I may consider that murder while you may consider it justice, who is right ? Without a law giver independent of human opinion it is all subjective.