1. Whenever we engage in moral activity we presuppose a moral norm. By moral activity I mean moral discourse, moral evaluation, and the like. When we say that "Brionna Taylor deserves justice", "Black Lives Matter!", "stealing is wrong", or similar statements we are engaging in these things. All of this presupposes a moral norm. Whenever we make a moral evaluation we suppose that there is some moral standard of judgment out there that tells us what's right and wrong and we are appealing to that.
When I say, for example, that "the fridge is broken - it ought not to be freezing the butter" I am appealing to a norm (a teleological norm). How do I know that it ought not be freezing the butter? I simply look at the manufacturer's guide to find out how the fridge ought to be working. The same happens in moral evaluation.
2. Relative norms depend upon absolute norms. Whenever we engage in moral activity we are actually presupposing not just any norm, but an absolute norm. Countries write laws and impose them on their citizens. Laws are a kind of relative norm because they are always subject to evaluation at a higher level. Just because something is a law, does not mean that it's good, just, or wise. We may always ask of any law: "Is this a good law? Is this a just law?" We can all think of example of unjust laws (Jim Crow laws, for example). But in order for us to evaluate any relative norm (like a law), there must be some absolute norm. An absolute norm is one that is not subject to evaluation at a higher level. We can ask: "Is this law good?" because there's something above the law whereby we may evaluate the law. Perhaps it's the constitution. Maybe when we ask: "Is this law good?" we are asking if it's constitutional. But then we may also ask: "Is our constitution good and just?" On and on this goes until we arrive at some absolute norm that cannot be evaluated at a higher level. If there is no such norm, we could never evaluate any relative norms at all. It would make no sense to ask: "Is this law good?"
3. Norms can only arise in personal contexts. Norms are only ever imposed by people. All relative norms that we know of are personal in nature. Behind every norm is a person or people who impose that norm. The fridge has a manufacturer that says how the fridge ought to work. The speed limit is imposed by a body of people. A nation's laws are imposed by people. Household rules are written and imposed by people. Every norm we can think of has a person or people standing behind it who have authority. It's very difficult to imagine an impersonal norm. What allegiance do we owe to the laws of physics, for example?
4. An absolute norm could only come from an absolute person. A norm that is not subject to evaluation at a higher level could only come from a person who is not subject to evaluation at a higher level - an absolute person. When we are talking about an absolute person, we are talking about something like God.
5. Therefore, whenever we engage in moral activity, we presuppose God's existence. If God does not exist there could be no absolute norms and thus no norms at all and all moral activity would be without meaning. Yet we find moral activity very meaningful. When we engage in it, we presuppose that God exists even if we resist this idea. We might simultaneously reject belief in God and accept belief in God while doing this.