There is no universal standard even with God. That's because a moral standard must exist in the form of human words.
So two questions:
1) What, exactly, are those words, and how do we know they come from God?
Usually proof of divine authority is provided in the form of some miraculous event. Like resurrection from the dead.
2) Who is the final judge of how those words are to be interpreted regarding particular instances of human behavior?
Each religious group have their experts which diligently study the words of prophets to provide a reasoned interpretation.
In secular matters, the answers are pretty clear. The words are those statutes, currently in force, as created and voted upon by the political authority under which one lives. They are interpreted, as regards any particular case, by a judge or jury duly authorized by the political authority.
Sure, pretty much the same. You can either accept the religious/political authority or reject it and face the consequences.
Something like that.
The problem with moral law is that we have thousands of prophets, each one proclaiming the actual law of God. And they don't even do it with any detail. Sometimes just a curt ten Shalt-Nots on a stone tablet. And then, a million preachers, priests, rabbis, etc., to give their opinions as to whether a particular behavior violates or doesn't violate the supposed law.
Moral law is a morass. Moral law means 'personal opinion of morality.'
I'm trying to get the idea of ethics and moral separate.
God's laws would be ethics since it is an external law (one that is imposed by an external force) vs morals which is an internal feeling of what is right and wrong.
So basically without God there is no authority that is superior to man to provide a standard for ethical behavior that all men would be expected to adhere to.
I think without that we are left with "might makes right".