How can we understand something that is not objectively true? At best, we can have an opinion, but it can never be fact.
My answer would be that we can't - at least not in the sense that you probably mean it. However, we probably need a few definitions before we can really dig into this. I'll say a few things about the underlined words, but since it's your statement, I'll let you offer whatever definitions best convey your intended meaning.
* Objective: IMO that's just a secular euphemism for "absolute". So, to my ears the statement sounds like you're making an appeal to absolute truth - something I expect you would say you don't believe. So, as we begin to wrangle some definition of "objective", I expect it to become a search for agreement on some foundational axiom.
* Truth: What started this whole thing off was my statement that truth can only be revealed. I'm not sure what we can accomplish by circling back to this.
* Fact: IMO a fact is agreement within a group that a particular statement about data will be accepted within that group. There is no claim of "truth" WRT facts, but only agreements of acceptance.
Then why do you propose that as the way to find out what is good and evil?
I'm not. That is the confusion here. Look at typical phrasings from the Bible. For example:
* He did what was evil
in the sight of the Lord ... (1 Kings 22:52)
* And you shall do what is right and good
in the sight of the Lord ... (Deut 6:18)
I don't care whether you call that absolute or relative morality. IMO it is what it is. If God says it, then that is the absolute and final word on the matter (though don't take that to mean I don't struggle with it or that I always comply perfectly). What that boils down to is that there is no such thing as good and evil (in an absolute sense). There is only God's will and whether I comply.
Good and evil, then, are only terms for a discussion between you and me. It's similar to facts. If we can agree something is beneficial, then that's good. If we can agree something is not beneficial, then that's evil. Those agreements, however, will be tough to come by when my touchstone is the Bible and yours is not.