“Now when they (Paul and Silas) had passed through Amphipolis and Apollonia, they came to Thessalonica, where was a synagogue of the Jews: 2 And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures,” – Acts 17:1,2
The Roman road, Via Egnatia, passed through Thessalonica and connected Byzantium to the region of Illyricum on the Eastern shore of the Adriatic Sea across from Italy. Today the region of Illyricum is part of the country of Albania. Paul very likely traveled the Via Egnatia as he journeyed to Illyricum.
” Through mighty signs and wonders, by the power of the Spirit of God; so that from Jerusalem, and round about unto Illyricum, I have fully preached the gospel of Christ.” – Romans 15:19
When Paul arrived in Thessalonica there was already a Jewish community there. For three sabbaths “he reasoned with them out of the scriptures”, most likely in the synagogue. Paul later wrote two letters to the church at Thessalonica. In the first letter, Paul commended them for their diligence in evangelism throughout the region of Macedonia and Achaia. Achaia encompassed the region to the South of Greece including Athens, Corinth, and the Peloponnese peninsula.
“So that ye were ensamples to all that believe in Macedonia and Achaia. 8 For from you sounded out the word of the Lord not only in Macedonia and Achaia, but also in every place your faith to God-ward is spread abroad;” – 1 Thes. 1:7,8
The Jewish population of Thessalonica surged in 1492 when Spain expelled their Jewish population. A fire in 1917 devastated the Jewish quarter. At that time the Jewish population exceeded 65,000 people. Interestingly, it was in 1917 that the Balfour Declaration set the stage for the establishment of the state of Israel.
During WW2 Greece was occupied by Germany. Between March and August 1943, more than 45,000 Jewish people from Thessalonica were shipped to Auschwitz where they died in the death camps. The once large and vibrant Jewish community was all but wiped out.
The only synagogue in Thessalonica that was not damaged during WW2 was the Monastir Synagogue. During the war the Red Cross used the synagogue as a warehouse.
Thessalonica’s old Jewish quarter. 
A flower vender near the mosque in central Thessalonica.
The Jewish Museum was opened in Thessalonica’s old Jewish quarter in 2001. It definitely merits a visit for anyone interested in Jewish history.