Is a “good” God really objectively better than a “bad” one? How do we know?

Matt, can you explain why is objectively true that moral being is always greater that a being that isn’t?

Sure, first let’s explore what it means for something to be objectively true.  When we say that something is “objectively” true we are saying that it is “mind independent”.  By contrast, if something is merely subjectively true then it is “mind dependent” (that is, dependent on the mind of the subject).
When it comes to morality, we will be asking the question, “Are things really good/bad or do I just like/dislike certain things?”  If moral truths are merely subjective truths, then by definition they are mind dependent.  Their rightness/wrongness is all in one’s own mind.  In that case, things like rape and murder are not really wrong but rather we merely find them distasteful.  Things like love and sacrifice would not really be “good”, it would just be true that we like those things.  A Jewish rabbi once said of this position, “You like milk, I like meat.  Hitler likes to kill people, I like to save them.”  What’s the difference?  If moral truths are merely subjectively true then they are no more “right” than one’s preference in food.
This is not at all, however, what we encounter in our moral experience.  In our moral experience we find that things like rape and murder are wrong, even if the perpetrators think they are right or permissible.  Every day we encounter objective moral values and duties (“rights” and “wrongs”, “goods” and “evils”) through our moral experience.  We are justified in believing what we experience firsthand, and are therefore justified in believing in objective moral values and duties.  That is to say, we are justified in believing that moral goods are objectively better than moral evils, that the difference between the two is not just in our own mind.
Since it is objectively true that moral goodness is better than moral evil, it will be objectively true that a moral being is greater than an immoral one.  God, as the greatest conceivable being, will be morally perfect.  He is the very standard or ideal to which all other things vary in degree from “good” to “evil”.

Matt Bilyeu
Chapter Director, Reasonable Faith
Master of Arts in Apologetics, Luther Rice University and Seminary