There is a long history of Christianity in Asia Minor dating back to the early church and the missionary journeys of the Apostle Paul. The last Christian Empire that ruled over Asia Minor was the Byzantine Empire but they were extinguished by the incoming Muslim Ottoman Empire. Particularly, Constantinople was conquered in 1453 by the Ottoman Sultan Mehmet II and renamed Istanbul. A visit to the Hagia Sofia Church partially explains what happened to Christianity across Asia minor as it was converted into a mosque and even many of the crosses etched in its stone walls were grinded off the wall. Upon entering Constantinople, the Ottomans slaughtered many residents, then ethnically cleansed much of the population and replaced it with their own people.
With that said, most cities in Asia Minor remained as Christian majorities under the Ottomans, especially early on. The Ottomans would build mosques or convert churches into mosques in each city in order to lure Muslim residents to the area. Many Muslims fled the Balkans to Asia minor in the late 1800s and early 1900s. Overtime, some Christian families converted to Islam but many remained Christian; particularly Greek Orthodox, Armenians, and Assyrians. These groups were allowed to function under the Ottoman millet system but they were regulated as dhimmis subjected to the Sultan’s rule and authority via Sunni Islam.
In the late 1800s and early 1900s, there were a series of massacres among Christian minorities which wiped out whole villages of Armenians and Assyrians. The Armenian Genocide between 1915-1917 is one of the sickest representations of how the Ottomans massacred Christians. Before WWI, there were around two million Armenians in the Empire but the majority were exported and hundreds of thousands were butchered. The Republic of Turkey and many Turkish scholars still do not acknowledge this genocide. Many Greek Christians and other Christians were exported from Turkey after the Turkish Republic came into being in 1923 through a mutual population exchange between Greece and Turkey where Greece gave Turkey their Muslims and Turkey gave Greece their Christians. There are still some Greek Orthodox in Turkey though, particularly Istanbul.
There is a small group of Protestant churches across Turkey who network together for the sake of the Gospel. The Lord is still at work in Asia minor and Christ will continue to build His church!