The Church of St. Dimitrios is the most important church in Thessaloniki and part of the city’s UNESCO World Heritage listing. There has been a church on the site since the 4th century, but the present structure dates back to 634 AD. It served as a mosque, Kasımiye Camii, between 1493 and 1912. The building was reconsecrated as a church in 1949 after sustaining damage during a terrible fire in 1917.
The remains of Άγιος Δημήτριος (St. Dimitrios) are inside. He was martyred in 303 AD by the Romans on the site of the church where the crypt is located. Inside the crypt is a museum. St. Dimitrios is the patron saint of Thessaloniki. Also inside the church are the remains of St. Anisia.
On one of my visits, I was lucky enough to be at the church on the feast day of St. Dimitrios. It was packed with people from all over Greece, Russia, Bulgaria, and other countries with a large Orthodox population.