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Orthodox Christians are grieving today, as a former cathedral is handed over to Muslims.
The Greek Orthodox archdiocese of America made July 24 a day of mourning after Turkey declared Hagia Sophia must be used as a mosque.
On Twitter Tuesday, U.S. Catholic bishops showed solidarity with the Orthodox, highlighting their day of mourning.
Hagia Sophia was a Christian cathedral built in Constantinople (modern-day Istanbul) in the sixth century, long before the schism between East and West.
It became a mosque after the city fell to the Ottoman Turks in 1453.
Nearly five centuries later, in 1935, it was transformed into a museum.
Though it was a mosque for centuries, there are accounts that a Greek military chaplain in World War I snuck in and offered Mass there.
Some see the reversion to a mosque as another way for Turkey — a predominantly Muslim country — to thumb its nose at neighboring Greece — a largely Christian country — as tensions flare over contested waters in the Mediterranean.
Turkish president Tayyip Erdoğan claimed Islamic prayers will resume in a matter of weeks.
The Islamic call to prayer will soon echo through the dome of Hagia Sophia as Christians around the world offer prayers of lamentation.