It’s often assumed by a lot of theists/Christians that objective morality does not exist without God; it’s almost taken as self-evident. Hence, it really seems to a lot of theists to be true. Objective morality is morality that would be binding whether or not human think so, and whether or not humans actually exist. This would mean that the proposition, “Murder is wrong”, is true independent of human thought.
But what argument or reason is there to think that objective morality doesn’t exist without God? Most of the time I just see it blindly asserted without rational justification. But in the context of debate, this is question-begging. In fact, there are a lot of theists who don’t think the proposition, “If God doesn’t exist, then object moral values don’t exist” is true. Some of these theists think the proposition is false, while other theists reject the proposition because they don’t see any reason to think it’s true.
Since the theist is the one running the argument, the theist has the burden of proof. Consequently, it won’t suffice to demand the non-theist give an ontological account of morality. The non-theist doesn’t have to give an account. The theist has to argue that the non-theist doesn’t have an account. All the non-theist has to do is rebut the theist’s reasons for accepting the proposition to be true.