"... but the righteous shall live by his faith." (Habakkuk 2:4 ESV)
This verse from Habakkuk had a tremendous influence on the life of Martin Luther. Luther encountered it in the monastery at Erfurt, although at first he was uncertain what it meant. Later he went through a dark period of illness and depression during which he imagined that he was under the wrath of God. Lying on a bed in Italy, and fearing that he was soon to die, Luther found himself repeating the words over and over again: "The righteous will live by his faith. The righteous will live by his faith."
Not long after he recovered, Luther went on to Rome, where he visited the church of St. John Lateran. The pope had promised an indulgence forgiving the sins of any pilgrim who mounted its staircase, which was alleged to have come from the judgment hall of Pontius Pilate. Believing that the steps were stained with blood of Christ, pilgrims mounted the stairs on their knees, pausing frequently to pray and kiss the holy staircase.
The story continues in the words of Luther's son, from a manuscript preserved in the library of Rudolstadt: "As he repeated his prayers on the Lateran staircase, the words of the prophet Habakkuk came suddenly to his mind: 'The just shall live by his faith.' Thereupon he ceased his prayers, returned to Wittenburg, and took this as the chief foundation of all his doctrine." Luther no longer believed that there was anything he could do to gain favor with God, and he began to live by faith in God's Son. As Luther himself later said, "Before those words broke upon my mind I hated God and was angry with him .... But when, by the Spirit of God, I understood those words--'The just shall live by his faith! The just shall live by his faith!'--then I felt born again like a new man; I entered through the open doors into the very Paradise of God."
(From Galatians: Reformed Expository Commentary, 111-112.)