There are many, just like there were in the congregations in Galatia, that are in the process of deserting Him who called them in grace. They may not be turning to a different gospel, but, they are turning away. Today, we have many gospels: the perverted prosperity gospel, the name it and claim it gospel, the I-had-a-moment-of-belief gospel and, my favorite, I-don’t-have-to-attend-a-service-or-be-active-as-a-Christian gospel.
These letters of Paul are sent for different reasons. The letter to the Galatian churches, to the congregations in Galatia, is one of rebuke and correction. Apostasy/rebellion was taking place, they were turning away from the true Gospel of the Kingdom, the Gospel of Jesus Christ.