You cannot say that Luther invented the idea of justification by faith alone. Long before Luther it was taught by Augustine and Paul and Jesus and Moses. Even … Adam and Eve realized soon after their sin that the fig leaves with which they tried to cover their shame were woefully inadequate. The gospel is given in Genesis 3:21 when Moses tells us that God clothed them. They needed something they couldn’t provide for themselves; and God giving man what man needs to stand in His favorable presence is the essence of the gospel. Luther merely restated what true Christians have understood for centuries, that justification is by faith alone.
John MacArthur, Jr
Martin Luther described the doctrine of justification by faith as the article of faith that decides whether the church is standing or falling. By this he meant that when this doctrine is understood, believed, and preached, as it was in New Testament times, the church stands in the grace of God and is alive; but where it is neglected, overlaid, or denied … the church falls from grace and its life drains away, leaving it in a state of darkness and death.
J.I. Packer
I can sympathize with Luther when he said, “I have preached justification by faith so often, and I feel sometimes that you are so slow to receive it, that I could almost take the Bible and bang it about your heads.”
C.H. Spurgeon
The courage to be is the courage to accept oneself as accepted in spite of being unacceptable. This is the genuine meaning of the Paulinian-Lutheran doctrine of justification by faith.
Paul Tillich