For they themselves report concerning us the kind of reception we had among you, and how you turned to God from idols to serve the living and true God, (1 Thessalonians 1:9 - ESV)
"They" ... the residents of Achaia and Macedonia that Paul has encountered are reporting to Paul, as he meets them, the way the Thessalonians received Paul and then responded to the message of the Gospel. Though turning from Idols is only mentioned here... This would have been a big deal for anyone from this time period. The real emphasis, as we will see, is the turn toward the living and true God.