Thessalonica was located (a) on the Via Egnatia, which crossed Greece from the Adriatic Sea in the west to the Black Sea in the east; (b) on the best natural harbour in the Aegean Sea; and (c) on the important land route linking the regions north of Macedonia with important cities (like Athens and Corinth) to the south. This meant that there would have been people passing through Thessalonica who were travelling in all possible directions to all the important cities in the Roman Empire. Disciples of Jesus who passed through would certainly have heard the news and repeated it excitedly in every Christian community to which they came.
8 For not only has the word of the Lord sounded forth from you in Macedonia and Achaia, but your faith in God has gone forth everywhere, so that we need not say anything.