What is Paul's constant warning in the book of Galatians?
Paul's constant warning in Galatians is that believers are not under Law but under Grace. He warns against Judaizers who would bring people back under the Law and pervert the Gospel by adding something to it. The letter was sent to congregations before it was too late.
Les explains that the Judaizers wanted to bring believers back under the Law, and this exercised the apostle to the point where he had to write this letter to those congregations before it would be too late. Les teaches that this little letter is appropriate today because believers face the same kind of bombardment. Many people do not like Paul's doctrines of grace because they want to do something and feel they have merited favor with God, but God will have none of it: either you believe He has finished it, or it will profit you nothing. So in Galatians' six chapters, the constant warning is that you are not under Law but under Grace, and you should not let these people pervert the Gospel by adding something to it. Les also notes that Galatians was probably written a couple of years before the Corinthian letters, and Paul's books in the New Testament are not in chronological order.