Ultimatum
If only God is good, how do you know which being is the right one to worship given two powerful ones? How do you pick, except either choosing the more powerful one and therefore being a slave to power, or choosing the one that fits your values better and therefore acting as a moral arbiter in God's place? Only God being good seems to fall apart as a coherent framework without it being backed by power, i.e. the most powerful being in the universe is just by default good because he can diddle your value function however he wants.