What did the Thessalonians turn from and to?
The Thessalonians turned from idols to the Living God. This conversion is cited by Paul as he contrasts it with the Galatians' drift from grace into legalism. They abandoned idolatry to serve the true and living God, and Paul marvels that others would move from grace back under the Law.
Les explains that Paul is writing to the Galatians, but he references the Thessalonians as an example of true conversion. The Thessalonians had turned from idols to the Living God. Paul says he is getting to the heart of the matter: the Galatian believers are being besieged by Judaizers who want to bring them back under the Law, circumcision, and temple worship. Paul marvels that they are so soon removed from Him who called them into the Grace of Christ, moving from the Gospel of the Grace of Christ into another gospel. He cannot comprehend how people who turned from idols to God would now be hoodwinked to go back under legalism.