Martin Luther on the Free Mercy of God

The Bible speaks of the free mercy of God in Jesus' unified righteousness by faith: 

"At last, by the mercy of God, meditating day and night, I gave heed to the context of the words, namely, "In it the righteousness of God is revealed, as it is written, ‘He who through faith is righteous shall live.’" There I began to understand that the righteousness of God is that by which the righteous lives by a gift of God, namely by faith. And this is the meaning: The righteousness of God is revealed in the gospel, namely, the passive righteousness with which merciful God justifies us by faith, as it is written, "He who through faith is righteous shall live." Here I felt that I was altogether born again and had entered paradise itself through open gates" (Preface to Luther’s Latin Writings).