John Chrysostom

"So that ye became ensamples to all that believe in Macedonia and Achaia." And yet it was later that he went to them. But ye so shone, he says, that ye became teachers of those who received (the word) before you. And this is like the Apostle. For he did not say, so that ye became ensamples in regard to believing, but ye became an ensample to those who already believed; how one ought to believe in God, ye taught, who from the very beginning entered into your conflict. "And in Achaia," he says; that is, in Greece. …

Homily on 1 Thessalonians 1 ↗

Theophylact of Ohrid

Although Paul came to them after others, nevertheless, he says, you shone so brightly that you became teachers of those who received the faith before you. And in this too is an imitation of Paul, since he also came last but surpassed everyone. Notice that he did not say: you will be an example for those who will believe, but: you have already "become an example" for believers of how one ought to believe — namely with fervor and readiness for dangers. By Achaia he means Greece.

Commentary on 1 Thessalonians ↗

Thomas Aquinas

And you are our imitators to such an extent that you can be imitated by others; therefore he says: "so that you become an example to all the believers in Macedonia and in Achaia." In making this point, Paul does three things. First, he shows that they can be imitated; secondly, he shows how their fame has spread (1:8); thirdly, Paul shows how they were praised by all peoples (1:9). …

Commentary on 1 Thessalonians ↗