Paul and Silas visit Thessalonica, and immediately upon arrival they rush to the synagogue to start arguing that Jesus is the Messiah. Of course.
Some Jews and Gentiles are persuaded, but another group of Jewish naysayers is not having it. They form a mob and hunt down Paul and Silas, as usual.
The naysayers are claiming that Paul and company are preaching against the Roman Emperor. After all, they are saying that Jesus is King. The emperor might not be too thrilled about that.
Paul In Beroea
Paul and Silas leave town and head to Beroea. The Jews and Gentiles are way warmer and friendlier there, and they enjoy listening to Paul's preaching.
The Jews from Thessalonica find out that Paul is in Beroea though, and they head on over to get some trouble brewing.
Paul In Athens
The Christians in town send Paul to Athens, where he'll be (a little) safer.
When he arrives, Paul is pretty sad to see that the whole city is covered in idols. Second commandment, everyone.
Naturally, he books it to the synagogue to start arguing with everyone about Jesus. Paul is consistent at least.
He moves on to the marketplace and starts to engage some of the non-Jewish Greeks in the crowd. He gives a sermon that's one part biblical and one part Greek philosophy.
Paul tells the citizens of Athens that they are obviously very religious (they have all those idols hanging around, after all), but God is the one who created the whole world and who should actually be worshiped. Oh yeah, and He also sent a guy (Jesus) who will lead the way—a guy who died, but was raised from the dead. Pretty awesome, right?
The Athenians are intrigued and tell him that they'd like to hear more. Some even become believers because Paul is just that darn good.