Morality, sans God’s order, is not simply made up, it is the result of the process of communication. In order to communicate with someone you have to see them as an intentional agent (so that you don’t think you’re talking to a rock), you have to believe that he or she is like you, a conscious being with the ability to freely learn, listen, take orders, and talk back to you. The key point here is that talking back, that they can disagree with you without being forced to do so. If they disagree, they can also agree without being forced to do so. If they can do that then, just as much as you think he or she might be wrong, the idea that you yourself might be wrong follows inevitably (It’s important to keep the distinction between talking to someone and talking with someone). It follows quite naturally from this process that if either of you can be wrong or right about something, that neither may be right or wrong, or that both might be right. From this realization, the potential for an objective ethical system is born.
This objective ethical system holds the same persuasive force as an Absolute system from God for it wasn’t made up my one person, neither was it made up by two people or by society democratically, but is inherent in the process of communication itself for with communication comes the ability to project yourself imaginitively into someone else’s place, to see that what you do to another is not in fact what you would want done to you.
It’s more complicated than that of course, but the gist of any ethical system comes from this simple, everyday event. It is not something made up, it is not something agreed upon, it is something that happens.
It’s not better than an Absolute system (historically and non-dualistically speaking, it turns out to be the same thing) for there is no compelling reason to do this, you aren’t forced to be a moral person, it is simply a possibility.
But what is the advantage of God’s system? Eternal punishment if you actions aren’t moral? One should be moral for fear of Divine Wrath? That doesn’t sound very Kantian to me.
And, to be honest, as described above, it is more of a legal system than an ethical system.