What IS good, and how do we determine it?

[Mister Agenda]

Some atheists who believe that objective moral right and wrong really do exist, a position also called moral realism, say they simply know that these moral truths exist, and that’s good enough for them. For example, Walter Sinnott-Armstrong answers the question, “Why is it wrong to cause harm without a good reason?” by saying, “It just is, don’t you agree?” Erik Wielenberg, in his book Value and Virtue in a Godless Universe, writes, “Of the ethical states of affairs that obtain necessarily, at least some are brute facts.”

Is it fair for atheists merely to assume that moral truths exist as unexplained brute facts? Luke Muehlhausesr, the founder of Common Sense Atheism, writes:

It's at least as fair as declaring whatever God says good because God said it, and God is good. All philosophies rest on at least one axiom that can't be proven and which you either find acceptable as a brute fact or you don't. The 'ultimate moral grounding' that so many Christians say they alone possess is not 'ultimately grounded' at all. They have to find the premise acceptable. They can't evade being ultimately responsible for their own ethics, since even handing over all their moral decisions to someone else is a decision that can only be reached by moral reasoning, whether sound or not.