Do the gods love the good because it is good, Socrates had wondered in Plato’s Euthyphro, or is something good because it is loved by the gods? Luther’s answer, like that of Muslim Traditionalists, was unambiguous. There was no rhyme or reason to God’s law. Humans had to accept God’s idea of the good simply because God tells us it is good, not because they could justify it through reason or through any external measure. Morality was indeed arbitrary. That was the whole point of
