Do you actually read any of these accounts before spouting off? It says he went in to reason with the Jews.
"Now when they had passed through Amphipolis and Apollonia, they came to Thessalonica, where there was a synagogue of the Jews. Then Paul, as his custom was, went in to [the Jews], and for three Sabbaths reasoned with them from the Scriptures, " (Acts 17:1,2)
The fact that there were gentiles there is a secondary considertion. When the Jews would not hear him in the synagogue, he went directly to the gentiles.
"And [Paul] reasoned in the synagogue every Sabbath, and persuaded both Jews and Greeks. ... But when [the Jews] opposed him and blasphemed, he shook his garments and said to them, 'Your blood be upon your own heads; I am clean. From now on I will go to the Gentiles.'" (Acts 18:5,6)
[Acts 17:4] Some of the Jews were persuaded and joined Paul and Silas, as did a large number of God fearing Greeks and not a few prominent women.
Why do you suppose we again see Gentile Christians worshiping in the synagogues with the Jews?
Why do you worship on the Catholic Sabbath?