But as Jaroslav Pelikan notes: Luther suddenly broke through to the insight that the “righteousness of God” that Paul spoke of in this passage was not the righteousness by which God was righteous in himself (that would be passive righteousness) but the righteousness by which, for the sake of Jesus Christ, God made sinners righteous (that is, active righteousness) through the forgiveness of sins in justification. When he discovered that, Luther said it was as though the very gates of Paradise had been opened to him.
