Why had the Galatian churches degenerated?
The Galatian churches had degenerated because a perverted Gospel had taken hold. The letter's greeting to the churches of Galatia subtly reveals how far they had already drifted, indicating that corruption of the Gospel was the cause of their decline.
Les explains that the phrase "all the brethren who are with me unto the churches of Galatia" is a subtle way of telling us how far these Galatian churches had already degenerated because of a perverted Gospel. He then points to Romans 1:7, where the believers in Rome are called beloved of God and saints, and to 1 Corinthians 1:2, where the church at Corinth is described as sanctified in Christ Jesus and called to be saints. These examples show the kind of commendation the Galatians lacked as a result of their corrupted Gospel.
And all the brethren which are with me, unto the churches of Galatia: