What was the ‘social setting” of the church at Thessalonica? Pollhill has a good summary of the usual arguments for the church being primarily Gentile (P&HL, 185). But this is problematic because Acts tells us that the congregation was formed after a period of time teaching in the Synagogue. In addition, Jews stirred up trouble for Paul out of jealousy – presumably because of his success in their synagogue.
The argument that the recipients of the letters are Gentiles rests on three observations. First, they are said to have turned “to God from idols.” Paul would not describe a Jewish convert as “turning from an idol.” Secondly, 1 Thess 4:1-8 describes some sexual ethics problems in the church. This would be more typical of a Gentile congregation than Jewish. Thirdly, 1 Thessalonians does not quote from the Old Testament, although 2 Thess seems to allude to the Hebrew Bible. If the church were written to a gentile audience with very little synagogue training and knowledge, we would expect few biblical quotations.
So where to these Gentiles come from? If the Gentile converts were God-fearers from the synagogue, then it is also unlikely that they would have “turned from idols” since they were worship God in the synagogue. In addition, a Gentile God-fearer might be expected to know as much of the Hebrew Bible as a Jewish person. The fact that the second letter is laced with allusions to the Hebrew Bible makes me think that there are other reasons for the lack in 1 Thessalonians. Paul was only in the city for a short time and there is no reference to evangelism in the marketplace, but he may have made contacts there which Luke chose not to report.
I think that the answer goes back to the persecution faced by the church. If they are persecuted for “rejecting Rome,” perhaps some of the “prominent people” Luke mentions in Acts 17:4 left the Christian church and returned to the synagogue, or to secular life. Those who remained “turned from idols,” specifically the national Roman cult. Someone like Jason was able to use wealth and power to deal with the court system in the city, so there is at lesat an implication that he was wealthy and connected politically. Perhaps Jason or other wealthy persons had left the church by the time Paul writes (suggested by Adolf Deissmann, c.f., Malherbe, 65).
The letter itself seems to praise the church for their strength in persecution, so maybe it is not wise to make too much of this alleged defection of some prominent converts, but it might explain the last of Jewish allusions in the letter.