If there's only good, then there is not free will. Without evil, good isn't a choice. A good follow up would be: Why is free will vital to God's plan? This game made me think more than I had intended. I'm not even religious.
In 10 Souls, you create the moral universe, designing souls and the world they live in. Use your unlimited power to grant freedom, encourage the good, and save souls from destruction. Or, condemn everyone to perdition. This is your world: You can design Good and Evil however you want.
There's no tutorial for working with God's machine. The only way to learn how it works is to play around with the levers. See if you can find a good answer to the question: If God is good, why is there evil?