Some commentators regard this letter as an abbreviated form of the "Gospel" (Good News) that St. Paul delivered in his letter to the Romans. There is some truth in this; but it does contain some valuable additions. Probably it was the original short form of the Epistle to the Romans.
After his introductory greeting, the Apostle deals directly with
a serious problem vexing the churches of Galatia. There were
some in the Galatian church who sought to undermine and mislead
the converts who accepted the "pure" gospel revealed
directly to the Apostle by Jesus Christ.
The problem centred around the question that still vexed some
sections of the church - whether Gentiles should be required to
accept the whole Law of the Jews before being accepted as Christians.
St. Paul repeats the teaching he gave in the letter to Rome of
"Justification by Faith".
Galatians 2.vv.15-21, and 4.vv.4-7
St. Paul, making a sharp distinction between the good principle
of "Spirit" and the evil principle "Flesh"
(unregenerate), describes the difference that living by the Spirit
makes in the lives of true believers.
Galatians 5.vv.19-25, and 6.vv.1-2, v.7-10, v.18