Can you be good without God? Godists and supernaturalists say no. They say that what is good is what God defined as good. If the ten commandments say "thou shalt not kill" then it is wrong to kill. Since most of us agree that it is wrong to kill, we should all agree, right?
The theory is, an action is good if God commands that it is good.
This kind of theory of morality is nice because it makes morals objective (have a truth value that is independent of what we think). We can resolve the differences between cultures by saying that there is only one right way of doing things, and that is what God commands.
Generally, arguments which rely on the existence of God are considered bad arguments. This is because the existence of God is controversial in itself. Nevertheless, a stronger way to attack such an argument is to assume that there is a God, and show that the argument doesn't work anyway. Socrates, an ancient Greek philosopher, thus managed to show how Supernalism fails as a theory.
Socrates asked: are actions good because God commands them, or does God command them because they are good? This question is very significant for Supernaturalism because it creates a dilemma.
If God commands that left handed people or blue eyed people are an evil blight then we would be right in exterminating them. If God decreed that we can only eat beets then we should. If the supernaturalist is right then these are distinct possibilities.
Some argue that God would not command such a thing. But what if he did?
Is God a slave to what is right? Can an omnipotent being not go against some other greater rule? If that is true than God is a shitty Deity. If He can than we cannot depend on this Deity for objective values.
Oddly, all the Supernaturalist I talk to use their own personal judgment to decide which of the Biblical commands to obey and which to ignore. If they can do that then why do they need the Bible to begin with? If they have the judgment then why not just use it?