RUTHFULLY YOURS

The Hagia Sophia has a dizzying history. It originally was built in 537 as the central cathedral of what would become Greek Orthodox Christianity. Ottoman Turkish Muslims conquered the Greek-speaking Christian Byzantine Empire and converted it into a mosque in 1453. But in 1934 Mustafa Kemal Atatürk, the founder and first president of modern Turkey, decreed Hagia Sophia would become a secular museum.

The symbolic meaning of the recent reconversion cannot be overestimated. Atatürk sought to substitute a secular, West-facing identity for Turkey’s traditional Islamic religious roots, which he saw as backward. A big part of that program was turning Hagia Sophia—for centuries a visual metaphor of Muslim triumphalism—into a museum. This had encouraged tourism and facilitated research by Western and Westernized scholars.

But Atatürk’s ambitious nationalism also created a Muslim monoculture. Millions of Greek Orthodox Christians and Armenian Christians had lived in Ottoman Turkey at the start of the 20th century. Genocide before and during World War I forced “population transfers” during Atatürk’s early presidency, and overt discrimination since then has reduced Turkey’s Armenian population to about 60,000. Only some 2,000 Greeks remain.

Atatürk’s secular culture flourished in cosmopolitan Istanbul and among Turkey’s educated elite. But it has barely penetrated the rural population, which today forms the base of Mr. Erdogan’s religiously conservative AKP party. A May survey from one Turkish newspaper showed 73% support across the country for reconversion, which has been under way for some time.

As early as 2010 the government began changing the building’s lighting to focus on its Islamic postconquest adornments. And in 2016 a muezzin chanted the Islamic call to prayer inside Hagia Sophia for the first time since 1934. “The secularists are beginning to understand what it’s like to be a religious minority,” says Elizabeth Prodromou, a Tufts University professor who researches the relationship between Orthodox Christianity and political Islam. Ms. Prodromou says Mr. Erdogan envisions Turkey displacing Saudi Arabia as leader of the Sunni Muslim religious world. “He is telling the Kemalists: Your interlude was a parenthesis.”

Hagia Sophia is still a sacred space for Christians, and their marginalization will be devastating for them. Completed under the direction of the Emperor Justinian, the building is an architectural marvel and Unesco World Heritage site. Its 150-foot-diameter dome seems to float above the building, thanks to a row of windows beneath it that flood the interior with natural light. Mehmet II’s invading troops looted and destroyed the church’s richly decorated icons and furnishings in 1453, and the sultans of the 17th century painted over its glittering mosaics depicting Christ, the Virgin, angels and saints, irreparably damaging many of them.

But until recently visitors still could experience the religious feelings of the Byzantine Greeks as they observed the light playing along the church’s marble floor and multicolored columns and facings. Bissera Pentcheva, an art historian at Stanford and author of “Hagia Sophia: Sound, Space and Spirit in Byzantium” (2017), has called the shimmering marble a kind of icon of the Holy Spirit “hovering over the primordial ocean.” Ms. Bissera spearheaded a project in which Stanford computer engineers virtually recaptured the building’s unique, highly reverberant acoustics. This enables the accurate recreation of exquisite Byzantine chants written specifically for Hagia Sophia.

When prayer rugs cover the marble floor, “the first thing that will suffer are the acoustics,” Ms. Bissera says. Obscuring the mosaics—whether by curtains, whitewashing or lasers—will further degrade any experience of the structure as the Christian edifice it was built to be. But Hagia Sophia isn’t the first to fall. Christian images have been obscured in other secularized Turkish churches turned mosques during the Erdogan era.

Turkey’s Muslim majority may be indifferent. The country is dotted with the ruins of its classical Greek past and nearly 1,000 years of Byzantine civilization—most of which have been deliberately destroyed or allowed to collapse. Atatürk’s experiment with secular and Western values seems to have come a cropper in a Turkey that takes religion more seriously than the secular West does. But for a structure like Hagia Sophia, it seems no change lasts forever.

Ms. Allen is the author of “The Human Christ: The Search for the Historical Jesus” (Free Press, 1998).