#srujansegev At the beginning of this third apostolic journey, around the year 53 (Acts 18:23), Paul passed through Galatia to visit the Christian communities he had established in the area (Acts 13:14ff), which he had also visited during his second journey (Acts 16:15). The communities in question were in the southern part of Galatia – Pisidian Antioch, Iconium, Derbe, and Lystra.
On this last journey Paul was very surprised to find that the Galatian Christians – most of whom came from a Gentile background—had been led astray by “false brethren,” Judaizers, who made out that Christians should conform to the Mosaic law and should be circumcised. Perhaps because he was short of time, the apostle was unable to go into the matter in detail; at any rate, when he reached Ephesus (53-54) he wrote the Galatians a letter, refuting the errors involved, in which he goes into the whole question of the relationship between the gospel and the Mosaic Law, between the Old Covenant and the New.
source