The beguiling Ayasofya Mosque in Istanbul started out as a Christian Orthodox Cathedral called Agia Sofia when Istanbul was named Constantinople.
Built in 537, it was converted to a mosque when the city fell to the Ottoman Empire in 1453.
When Ataturk founded the new secular Turkey a few years after WWI, the mosque was turned into a museum until controversially the current President Erdogan restored it to mosque status in 2020, a move that upset Eastern Orthodox Christians everywhere.
Internally it features many fine Byzantine icons and other fine architecture. There is a continuing concern that one day these could be damaged or destroyed. There cannot be many mosques with Christian artefacts inside (The Mezquita in Cordoba in Spain is another).