Lutherans believe that salvation is a free gift of grace from God. The Bible speaks on grace:
Romans 3:23: All have fallen short of the glory of God; they are now justified by his grace as a gift, through the redemption that is in Christ Jesus.
Ephesians 2:4-5: But God, who is rich in mercy, out of the great love with which he loved us even when we were dead through our trespasses, made us alive together with Christ—by grace you have been saved.
John 1:16-17: From his fullness we have all received, grace upon grace. The law indeed was given through Moses; grace and truth came through Jesus Christ.