For 1,500-years, Hagia Sophia, once the greatest religious building in the Christian world, has held a deep significance for the people of what is now known as Istanbul (formerly Constantinople). Completed in 537 A.D. by the Byzantine Emperor Justinian I, the complex became the seat of the Eastern Orthodox patriarch and the site of Byzantine imperial ceremonies.

Hagia Sophia spent more than 900 years as the navel of the world for Eastern Orthodox Christians and as an inspiration for architects and artisans employed by the Doges of Venice, marauding Crusaders from Western Europe, the Holy Roman Emperors of Aachen, and the princes of Kievan Rus.

In 1453, the Ottoman Empire under Sultan Mehmed II captured Constantinople. The new Muslim ruler of what had been the capital of the Byzantine Empire insisted that the church needed to be renovated and converted into a mosque.

https://www.neweurope.eu/article/the-eu-and-unesco-should-not-allow-hagia-sophia-to-be-captured-by-turkeys-religious-nationalists/