What does Romans 5:21 mean?

Romans 5:21 brings to a climax the argument Paul has been building throughout the chapter, and really since chapter 1. In plain language, the verse says this: sin once held humanity in a kind of tyranny, a rule whose ultimate outcome was death. But now grace has taken up a rival rulership, and its outcome is not death but eternal life, granted through righteousness, and made possible entirely through Jesus Christ. Paul is picturing two competing kingdoms, or two reigning powers, one leading downward into death, the other leading upward into life, and he is announcing which one has won.

To understand the context, remember that Paul is writing to the church in Rome, a congregation he had not founded but hoped to visit, made up of both Jewish and Gentile believers still working out what it meant to live under grace rather than under the Mosaic law. In the verses just before this one, Paul has been comparing Adam and Christ. Through Adam's one act of disobedience, sin entered the world and death followed as its inevitable companion, spreading to all people. Paul's rabbinic-style argument in 5:12-21 is often called the "Adam-Christ typology": Adam is a pattern, a foreshadowing, of Christ, but Christ's act of obedience reverses and overwhelms what Adam's disobedience set in motion. Verse 21 is the summary sentence, the "so that" clause explaining the purpose of grace's abundant increase mentioned in verse 20: where sin increased, grace increased all the more, precisely so that grace could establish this greater reign