Luther was held in darkness by the devil, though he was a monk. He was trying to save himself by works. He was fasting, sweating, and praying; and yet he was miserable and unhappy, and in bondage. Superstitious Roman Catholic teaching held him captive. But he was delivered by the word of Scripture—“the just shall live by faith.” From that moment he began to understand this Word as he had never understood it before, and the better he understood it the more he saw the errors taught by Rome. He saw the error of her practice, and so became more intent on the reformation of the church. He proceeded to do all in terms of exposition of the Scriptures. The great doctors in the Roman church stood against him. He sometimes had to stand alone and meet them in close combat, and invariably he took his stand upon the Scripture. He maintained that the church is not above the Scriptures. The standard by which you judge even the church, he said, is the Scripture. And though he was one man, at first standing alone, he was able to fight the papal system and twelve centuries of tradition. He did so by taking up “the sword of the Spirit, which is the word of God.”
from The Christian Soldier, Martyn Lloyd–Jones