He is known for launching the Protestant Reformation in Germany. His “tower experience” convinced him that the essence of the gospel is that justification comes only by the gift of God’s grace appropriated by faith (sola gratia; sola fide). According to Luther, God declares the sinner righteous through Jesus’ death rather than through human merit or works. Faith entails trust in and acceptance of God’s gift of salvation through the “merits” of Christ (from Pocket Dictionary of the Theological Terms).
To learn more about him, you can read “Martin Luther and Why He Matters.”