Thanks for the intelligent post. Moral relativists are difficult opponents. Once they have rejected truth -- not necessarily
the truth which we all struggle to understand and pursue but rather that there is truth -- they inevitably begin contradicting themselves. Reason breaks down and they fail to see it, so that self-contradiction becomes inevitable and habitual. And because they are accustomed to it, they do not admit. They become blind.
Now we are all blind. And no one, aside from God, has complete knowledge of the truth. But because we do not understand the truth fully we are criticized as truth-mongers. I have never claimed to know the truth completely. I associate myself most closely with the Socratic-Platonic approach to truth: I know that I do not know, and yet that doesn't mean that I don't have a few very good opinions verging on truth. And I also have that light of reason, Faith: what God has revealed concerning matters important for men to know but as Aquinas puts it: we err for two reasons, sin and ignorance.
And I thank you for all of your intelligent, incisive posts, too. I believe with you that we can know truth truly without knowing it exhaustively. It's like a young child who can truly know his mother or father without knowing every last thing about them.
Cordially,