Martin Luther (1483-1546 AD) became an Augustinian, Roman Catholic monk where he studied the Word of God diligently while still in the monastery. His study convinced him to post his 95 theses, statements he wanted to debate within the context of the Church to restore it. The rest is history as Martin Luther was excommunicated from the Roman Catholic church and became the first "Lutheran."
Luther was convinced that: God justifies a person (declares him righteous and acquits him) by faith alone and not by works, each believer has access to God directly apart from any human intermediaries, and the Scriptures are the true source of authority for both faith and life. Many of his doctrines, especially on justification, he covered brilliantly in his commentary on Galatians. And rightly so, for Galatians was his favorite book, his "Katherine," and it was central to his understanding of the gospel.
Luther's Commentary on Galatians in the history of the Christian Church is very remarkable. It presents like no other of the central thought of Christianity: the justification of the sinner for the sake of Christ's merits alone. Luther also delineates the difference between Law (what God demands from us) and Gospel (what God has done and does for us); in this text, we understand his "simul justus et peccator," that is, a Christian is simultaneously 100 % saint and 100 % sinner.
To understand Christian theology and justification by faith, reading this commentary is proper, right, for our eternal good--for Luther explains the doctrines of the Scriptures in forthright boldness and clarity.