 First of all, he says sin is what causes death. That's what he says in verse 56. The sting of death is sin. Death was a penalty for sin. Adam sinned. We all received the death penalty. So sin is what causes death. Secondly, he says the law is what establishes sin. The law of God shows us that every person is guilty before God. It shows us that none of us have measured up to the righteous standards of God. It shows us that everyone deserves the wrath of God.