Atheists who announce "I'm good without god"

[Angrboda]

Physical laws can't give you an ought.   They are what they are.   You combine X and Y and get Z.   They cannot tell you, that it ought to give you "A".    For this you would need a personal transcendent authority which can say that it ought to be A, even though it is Z.  In addition, with God as the moral authority, it is not arbitrary, as morality is based on God's nature, which is unchanging and maximally great. 

What do you mean by maximally great here?