From 1510 to 1520, Luther lectured on the Psalms, the books of Hebrews, Romans, and Galatians. As he studied these portions of the Bible, he came to view the use of terms such as penance and righteousness by the Catholic Church in new ways. He became convinced that the Church was corrupt in its ways and had lost sight of what he saw as several of the central truths of Christianity, the most important of which, for Luther, was the doctrine of justification—God's act of declaring a sinner righteous—by faith alone through God's grace. He began to teach that salvation or redemption is a gift of God's grace, attainable only through faith in Jesus.