In 2017, the year of the Reformation anniversary, the four basic evangelical principles are linked in many ways: Christ alone - scripture alone - by grace alone - in faith alone. For Luther, the fundamental insights of the Reformation had their biblical basis in the letter to the Galatians: "The letter to the Galatians is my 'little epistle', in which I have trusted myself, she is my Käthe von Bora" (WA 1, no. 146 [1531]). Paul's letter to the Galatians is one of the most important writings in the New Testament. The apostle writes to congregations that came into being through his preaching of the gospel of Jesus Christ, but which, from Paul's point of view, are in danger of losing their foundations and straying from the "truth of the gospel" due to current developments. The heartfelt solidarity, the struggle for the binding truth and the deep concern for the well-being and salvation of his communities determine the special intensity and personal passion of Galatians. At the same time, Galatians is a rich source for central themes of the gospel. The meaning of the coming of Jesus Christ up to his life sacrifice on the cross, the questions of the origin of the gospel, the justification of man before God, the limited function of the law as well as the connection between God's promise in Scripture and its fulfillment in Christ are developed in detail. We learn of the unique significance of God's grace shown in Christ and of faith grounded in the gospel, of the overwhelming experience of Christian freedom and new life in the Spirit of God. Finally, Galatians is also one of the most important sources for our historical knowledge of Paul's life and work and for our understanding of the early Christian communities. For in Galatians we are dealing with a very early testimony of an eyewitness and champion of the spread of the gospel in the world of that time. An explanation that is as understandable as it is profound, which introduces the proclamation and thinking of the Apostle Paul and invites renewed questions about the "truth of the gospel" and about the "freedom of the children of God".