Turkey’s Decision to Turn Hagia Sophia Into a Mosque Dismays Christians, Neighbors, Historians

ISTANBUL—With its vast dome and towering minarets, Hagia Sophia is to humanity what the Grand Canyon is to geology: an open-air book that concentrates nearly one and a half millennia of history as the successive heart of Orthodox Christian and Muslim empires, punctuated by ransacking, fires and earthquakes.

But by deciding on Friday to transform back into a full-fledged mosque a building that has been a museum since the 1930s and has become a powerful symbol of ecumenical relations, Turkish President Recep Tayyip Erdogan sent…