Let’s say you’re in a Chinese food restaurant. At the end of your meal, you crack open the fortune cookie, and it says, “Get up, and leave now.” How many of you would get up and leave? Would you feel obligated to obey that command? I don’t think anyone would.
Let’s say the lady next to you says, “Get up and leave now.” You ask, “Who are you?” She says, “I’m an FBI agent. I’m under cover, and we’re about to bust the owner of this restaurant. Get up, and leave now.” Do you get up and leave? Yes, you do. There now seems to be an appropriate obligation to follow that command.
Obligations only make sense in the context of persons. Not just any persons. There has to be one person that has appropriate authority to issue commands. Moral obligations, the theist would argue, only make sense in the context of the existence of God.