Mardin was once home to a large Christian community and several churches still remain in the city, although only a few are functioning. The most likely to be open is the Kirklar Kilisesi, an Assyrian Orthodox church. It's on a narrow lane a hundred yards or so west of the Mardin Museum. Parts of the church dates back to the year 569, though most of what you see is medieval. There are some beautiful stone carvings and a shady courtyard. Neighborhood children will offer to take you there, which is probably a good idea, since it can be hard to find.
217 Sağlik Sok. 8, Mardin, Mardin, Turkey