The first letter to the Thessalonians is generally taken to be the earliest surviving Christian writing, composed around 50 CE, which means the oldest document of the religion is a pastoral note to a congregation in this city. Its immediate problem is a practical one that only arises if you expect the world to end soon: members of the community have died, and those left behind want to know whether they have missed what was coming. Paul answers that the dead will not be at a disadvantage. Acts describes his brief and stormy visit, ending with his hosts dragged before the magistrates.
The city's own patron came later. Demetrius, remembered as a soldier or an official killed in the persecutions of the early fourth century, acquired a great basilica over a crypt, and by the middle ages his tomb was reported to give off a fragrant oil that pilgrims carried away in small flasks. He was credited with defending the city against sieges, and mosaics in the church show him with the officials who built it.
Thessalonica was the second city of the Byzantine empire, and its churches trace the whole history of eastern Christian architecture, from the rotunda that Galerius built as a monument and that was converted first to a church and later to a mosque, through the great basilicas, to the small painted churches of the fourteenth century.