Hagia Sophia, which means "Holy Wisdom," was built in 537 under the Byzantine Emperor Justinian. For a time it was the largest building in the world and the largest Christian church. It served as the cathedral of the Patriarch of Constantinople before and after the Great Schism split Western and Eastern Christianity into Catholic and Eastern Orthodox Churches.
After the Ottoman capture of Constantinople in 1453, the cathedral was converted into a mosque. Under the Ottomans, architects added minarets and buttresses to preserve the building, but the mosaics showing Christian imagery were whitewashed and covered.
In 1934, under a secularist Turkish government, the mosque was turned into a museum. Some mosaics were uncovered, including depictions of Christ, the Virgin Mary, John the Baptist, Justinian I, and the Byzantine Empress Zoë Porphyrogenita.
It was declared a World Heritage Site under UNESCO, the United Nations Educational, Scientific and Cultural Organization. When the museum becomes a mosque, it is believed that the mosaics will have to be covered during Muslim prayers, as well as the seraph figures located in the high basilica dome.
Turkey's population of 82 million is overwhelmingly Sunni Muslim. Non-Muslim minorities make up only 0.2%, and the Christian population is split among several Orthodox and Catholic Churches, as well as other groups.