Determining that "good" means anything other than "of God's nature" is circular.
Although this was addressed to Theocrat, I would like to jump in here on this point and I believe that Theocrat would agree with my perspective.
When anyone attempts to establish an ultimate authority or standard, circular reasoning becomes unavoidable. By definition, ultimate authorities are just that, ultimate authorities. If you appeal to something else to prove your ultimate authority, what you used to prove your ultimate authority then becomes your ultimate authority.
For the Christian, God and His self verifying word is the ultimate authority and with this being the case, the highest validation that the Bible can possibly have is God’s own word. So for sake of argument, if the Christian God exists, the ultimate proof for His existence can only be His own word.
Now if your criterion for establishing sound reasoning is that no one can engage in circular reasoning even when dealing with ones ultimate authority, then you must exclude everyone’s reasoning. For example; for most atheists, logic and reason are the ultimate authority, now does he use logic and reason simultaneously while trying to prove that logic and reason are his ultimate authority? The answer is obviously yes, because if he does not use logic and reason when doing so, he just becomes a blithering idiot.
So the real question that needs to be answered is not whether someone is using circular reasoning when trying to establish their ultimate authority but whose ultimate authority makes all other ultimate authorities intelligible. This is where the Christian position can be demonstrated to be the superior view.
You might be asking, how is the Christian position superior. But before I demonstrate this, I would like to expand on what I mean by intelligible. By intelligible, I mean that it “makes sense” in a particular persons world view. IOW, it means it fits what that person says about what exists (their metaphysic), how they know what they know (their epistemology) and how they should act as a result (their Axiology).
Now in the examples I’ve used, the atheist position can not make sense of why we ought to use logic and reason to resolve our differences, whereas in the Christian view, it makes perfect sense as to why we should be kind and reason with each other and not use force to get our way in life, since doing so would violate the ultimate standards that God has given us in His word. When tyrants come by force to take what is the atheists’, it does not make sense for him to appeal to any invisible, absolute, invariant, abstract laws like laws of morality since according to his view of reality, these kinds of things do not exist.
Most atheists are unwilling to follow the logical conclusions of their chosen view of reality although people like Fredrick Nietzsche, David Hume and Bertrand Russell were willing to concede many of the points I am making in regard to things like laws of nature and morality.
So even when it comes to our most fundamental assumptions of life that we as human beings all take for granted, the atheist has no rational basis for holding to any of these assumptions, he secretly has to borrow the intellectual currency of the Christian in order for his argument to make sense given his view of reality.
Therefore, the Christian may reason in a circle, but it is not the kind of vicious circle that the unbeliever must rely on. The Christians circular reasoning makes reasoning itself and all other abstract things that humans experience, such as love, beauty, human dignity, morality etc. intelligible.
"we are not getting rid of God because we still believe in grammar" Frederick Nietzsche