This is an adaptation and extension of an argument by Plato, in the dialogue Euthyphro.
Let us say, as many do, that the definition of The Good is nothing more and nothing less than the Will of God. That would mean that whatever world God creates would be, by definition, perfectly good. Our world, then, is not the best world God could have created – any world must, by definition, be perfectly good. That means that a world far more miserable than ours, e.g., one where suffering is not justified, one in which there is no prospect of salvation, must be equally good as ours, if God were to decide to create such a world. Equally, for those who believe that all morality requires God as moral arbiter, then whatever morality God decides is objectively moral, even if it were the opposite of justice and morality in our world, even if it would promote dishonesty, selfishness, unjust privelege and liability, etc.
In such a case, though we may be beholden to God and subject to His reward and punishment, there is nothing “good” about the world or about moral behavior as dictated by the command of God. That is to say, though perhaps we would be grateful that God created us at all, we should not be grateful that He created a world for us that is good. Any world God would create is good by definition.
But the faithful believe that not only is God the creator, not only does He write the rules of the game, but that the rules are fair, just, and good.
Conclusion: There is a criterion for what counts as good independent of God and God’s will. Thus, if God is truly good, He is good according to a criterion He does not Himself create.