Paul’s Letter to the Galatians is likely his earliest letter in the New Testament and his most stern letter.  He is upset that people in the churches in Galatia are turning from a gospel of “grace through faith” to a different gospel of “grace through faith plus obedience to the Law of Moses” in order to be saved.  This is an intense presentation of the Gospel.  This is the book that turned the heart of Martin Luther to salvation and became his constant companion when preaching the gospel during the Reformation.