This is a shock letter! Paul wrote this letter. He is astonished over the report that the Galatians were being adulterated by the so-called Judaizers who would convince the Gentile believers to be circumcised and lead their life by the law. Paul is schocked! Observe:
I marvel [or astonished!] that ye are so soon removed from him that called you into the grace of Christ unto another gospel.
– Galatians 1:6.
Note he calls that message "another gospel". It is another gospel and not the true gospel of Christ which he taught them. So to take his shock/amazement to another level, he notes:
…but there be some that trouble you, and would pervert the gospel of Christ. But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
– Galatians 1:7, 9.
He calls an anathema (i.e., a very severe term in Greek calling a curse) upon them that bring or teach another gospel. Even if an angel come down from heaven with another gospel that is not of Christ, yes, even the angel, let them be anathema, accursed!
The apostle therefore goes to address the Judaizers that it is impossible for one to keep the law, just as their forefathers failed to do. None can be justified by the law. Even in explaining these teachings, Paul is sound and not rude. He ends the letter with some practical exhortations for Christian living.
The letter was written around, c. A.D. 48/49-51. The key word is, Law (Gk. nomos). The key verse that ties the message of the whole book is,
We…know that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.
– Galatians 2:15, 16.
Broken For Christ 
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