The conversion, by President Erdogan of Turkey, of the Church of the Holy Saviour at Istanbul into a mosque is a moment to mark. It’s another reminder that the demesne of Atatürk is tilting again away from the West. Though one could say that the fate of the churches was sealed in 1453, when the Ottomans conquered Constantinople, Mr. Erdogan’s escalating hostility to Israel and Byzantium’s past glories spells trouble on the Bosphorus.

Mr. Erdogan’s Islamizing of the church, whose origin dates to the fourth century and whose walls are adorned with 14th century frescoes, drew protest from Greece’s prime minister, Kyriakos Mitsotakis. Like the Haghia Sophia, first built by the Emperor Justinian, this house of worship was built as a sanctuary for Christians, reworked into a mosque, turned into a museum in World War II’s wake, and now is a mosque again.