What does Romans 5:21 mean?

Previously, Paul has written that one result of Adam's devastating sin was that death came to reign over the lives of humanity (Romans 5:17). Now he writes that death's reign was, in truth, the reign and rule of sin. Sin brings death. Human beings cannot escape our own sin nature no matter how badly we want to. In ourselves, without God's intervention, we will always continue to sin. It rules over us, and it leads to death.

However, Paul has just written that as sin increases, God's grace increases even more. In other words, sin cannot grow past God's capacity to give good to those who deserve His angry judgment instead. Paul concludes that God's grace is the greater ruler. It reigns over sin and death. How? He declares righteous all of us sinners who, by faith, receive his grace-gift of Jesus' death for our sin on the cross.

With death defeated, those who are in Christ will live forever. Grace reigns through righteousness leading to eternal life. But this eternal life is found only through Jesus Christ our Lord. There is no other way to escape the reign of sin and death.

Starting with the next verse, Paul will counter a common and unfortunate response to this idea of abundant grace. Some see God's grace as a license to sin, which is not what God intended, nor what Paul is teaching. The overflowing mercy of God is not a reason to sin, just so that His grace can increase.