From 1510 to 1520, Luther lectured on the Psalms, and on 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 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 as the Messiah. "
This one and firm rock, which we call the doctrine of justification", he wrote, "is the chief article of the whole Christian doctrine, which comprehends the understanding of all godliness."

Luther came to understand justification as entirely the work of God.
This teaching by Luther was clearly expressed in his 1525 publication On the Bondage of the Will, which was written in response to On Free Will by Desiderius Erasmus (1524).
Luther based his position on predestination on St. Paul's epistle to the Ephesians 2:8–10.
Against the teaching of his day that the righteous acts of believers are performed in cooperation with God, Luther wrote that Christians receive such righteousness entirely from outside themselves; that righteousness not only comes from Christ but actually is the righteousness of Christ, imputed to Christians (rather than infused into them) through faith.

"That is why faith alone makes someone just and fulfills the law," he wrote. "
Faith is that which brings the Holy Spirit through the merits of Christ."
Faith, for Luther, was a gift from God; the experience of being justified by faith was "as though I had been born again."
His entry into Paradise, no less, was a discovery about "the righteousness of God"—a discovery that "the just person" of whom the Bible speaks (as in Romans 1:17) lives by faith.
He explained his concept of "justification" in the Smalcald Articles:
The first and chief article is this: Jesus Christ, our God and Lord, died for our sins and was raised again for our justification (Romans 3:24–25).
He alone is the Lamb of God who takes away the sins of the world (John 1:29), and God has laid on Him the iniquity of us all (Isaiah 53:6).
All have sinned and are justified freely, without their own works and merits, by His grace, through the redemption that is in Christ Jesus, in His blood (Romans 3:23–25).
This is necessary to believe.
This cannot be otherwise acquired or grasped by any work, law or merit.
Therefore, it is clear and certain that this faith alone justifies us ... Nothing of this article can be yielded or surrendered, even though heaven and earth and everything else falls (Mark 13:31).

Luther's rediscovery of "Christ and His salvation" was the first of two points that became the foundation for the Reformation.
His railing against the sale of indulgences was based on it.
From 1510 to 1520, Luther lectured on the Psalms, and on 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 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 as the Messiah. "This one and firm rock, which we call the doctrine of justification", he wrote, "is the chief article of the whole Christian doctrine, which comprehends the understanding of all godliness."
Luther came to understand justification as entirely the work of God. This teaching by Luther was clearly expressed in his 1525 publication On the Bondage of the Will, which was written in response to On Free Will by Desiderius Erasmus (1524). Luther based his position on predestination on St. Paul's epistle to the Ephesians 2:8–10. Against the teaching of his day that the righteous acts of believers are performed in cooperation with God, Luther wrote that Christians receive such righteousness entirely from outside themselves; that righteousness not only comes from Christ but actually is the righteousness of Christ, imputed to Christians (rather than infused into them) through faith.
"That is why faith alone makes someone just and fulfills the law," he wrote. "Faith is that which brings the Holy Spirit through the merits of Christ." Faith, for Luther, was a gift from God; the experience of being justified by faith was "as though I had been born again." His entry into Paradise, no less, was a discovery about "the righteousness of God"—a discovery that "the just person" of whom the Bible speaks (as in Romans 1:17) lives by faith. He explained his concept of "justification" in the Smalcald Articles:
The first and chief article is this: Jesus Christ, our God and Lord, died for our sins and was raised again for our justification (Romans 3:24–25). He alone is the Lamb of God who takes away the sins of the world (John 1:29), and God has laid on Him the iniquity of us all (Isaiah 53:6). All have sinned and are justified freely, without their own works and merits, by His grace, through the redemption that is in Christ Jesus, in His blood (Romans 3:23–25). This is necessary to believe. This cannot be otherwise acquired or grasped by any work, law or merit. Therefore, it is clear and certain that this faith alone justifies us ... Nothing of this article can be yielded or surrendered, even though heaven and earth and everything else falls (Mark 13:31).
Luther's rediscovery of "Christ and His salvation" was the first of two points that became the foundation for the Reformation. His railing against the sale of indulgences was based on it.
Martin Luther