Is morality objective with God? If so, why?

Something many theists point to is without God, morality is subjective. For example, an atheist can't justify that murder is intrinsically wrong. If God does exist however, then morality *is* objective and murder *is* intrinsically wrong. On the surface this makes sense, but why is this actually the case. A government or authority figure can't justify intrinsic moral values, but if God exists then *he* can? Even if God does exist, there is still opinion on whether we should listen to him or not (e.g. Lucifer's position).