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. 

The fate of the church that the historian John Julius Norwich called the “Sistine Chapel of Istanbul” is of a piece with a trend our Hillel Halkin discerned as far backs as 2007, when he wrote that the “Islamic counterrevolution has won the day in Turkey” and reckoned that “secular Turkey, at least as Atatürk envisioned it, is a thing of the past.” Mr. Halkin previously broke the story that Atatürk’s father was a member of a Jewish sect. 

Of more immediate consequence than even Mr. Erdogan’s erasure of the memory of New Rome is his growing belligerence to the Jewish state. He calls Hamas a “resistance” movement and declares that he is following them “step by step, and there are more than 1,000 Hamas members who are all being treated in our hospitals. This is how we are doing things.” Mr. Mitsotakis’s response to all that was “let us agree to disagree.”

Mr. Erdogan has halted all trade with Israel, Air Turkey is no longer landing at Ben Gurion Airport, and on Sunday Turkey’s president ventured that Prime Minister Netanyahu “has reached a level that would make Hitler jealous with his genocidal methods.” He supports what he takes as Hamas’s goals — to “take back the Palestinian lands occupied by Israel and restore their state.” Some attribute the hostility to a need to inflame Turkey’s voters.

Turkey, a NATO ally, not long ago enjoyed civil relations with Israel. President Herzog even traveled to Ankara for a state visit. That made some repairs to the damage caused by the Mavi Marmara flotilla. Mr. Halkin attributes to Atatürk’s Jewish roots his “iron-willed determination to create a strictly secular Turkish nationalism.” The turn against the Jews — and Turkey’s Christian heritage — suggests the project could soon be put paid.