Art historian Andrew Gould had studied many copies of the exquisite mosaic of Jesus found high in Istanbul's 6th Century Hagia Sophia cathedral.

But that didn't prepare the architect and sacred artist for what he felt when he stood under the icon, illumined by the soaring windows in the south gallery that overlooks the main floor, under the central dome that is 184 feet high and 102 feet in diameter. 

The Deesis ("supplication") icon -- at least twice the size of life -- shows the Virgin Mary and St. John the Baptist with their heads bowed, framing an image of Christ Pantocrator ("enthroned"). The glass mosaic cubes were set at angles to create a shimmering effect across the gold background and the many-colored images, whether viewed in daylight or with lamps and candles.

Much of this icon was destroyed a century ago as workers probed to find priceless mosaics under layers of plaster and paint added through the centuries after 1453, when the Ottoman armies of Mehmed II conquered Constantinople.

Now, Turkish leaders want to convert Hagia Sophia -- a museum for decades -- back into a mosque.

"There is no more refined icon of Christ anywhere," said Gould, of the New World Byzantine Studios in Charleston, S.C. "Just in terms of information, we have copies we can study. … But visiting Hagia Sophia and seeing this icon under natural light, seeing it in the context of the sanctuary, was crucial to the development of my whole understanding of Orthodox art."

If the "Deesis" is covered again, along with other icons, "this is not something that can be replaced with photographs in art books," he said. "It would hurt artists and believers around the world in so many ways."

The current controversy is rooted in politics, more than lingering tensions between Muslim leaders and Turkey's tiny Christian minority, which has little power other than through ties to Greece, Europe and the United States.

Hagia Sophia became a museum in 1934, a symbol of Mustafa Kemal Ataturk's drive to build a modern, truly secular state. Now, President Recep Tayyip Erdogan has sent many signals that he wants Turkey to return to Islamic principles.

Debates in Turkish media have swirled around whether modern leaders retain the "right of the sword" to reclaim Hagia Sophia, noted Mustafa Akyol, a Turkish journalist and author of the book "Islam Without Extremes: A Muslim Case for Liberty."

This act would be a "silver bullet" in the Turkish leader's efforts to retain power amid charges of corruption and brutal authoritarianism after nearly two decades in power, he said. The current debates follow Muslim prayers in Hagia Sophia in 2016 and, in 2018, rites led by Erdogan himself in memory of Mehmed II.

"If he reopens Hagia Sophia to Islamic worship, he thinks he will be doing something historic that Turkey's conservatives will honor for many years, if not decades," said Akyol, a senior fellow at the Cato Institute in Washington, D.C. "Some opposition parties support him on this idea, because they don't want to look insufficiently nationalist or Islamic. …

"It is one thing if Muslims merely worship inside the building. … It is another thing if they decide to cover all the images of Jesus Christ and Mary, because human images are forbidden in mosques."

Turkish news reports and social-media exchanges have signaled rising tensions, including a claim that purple prayer carpets have already been purchased for Hagia Sophia.

One Islamist leader claimed, on Twitter, that there is no need for a formal decision by Turkish officials, since "we've already purchased the lime" for plastering. Abdullah Sevim of Saadet Partisi also tweeted: "I am looking forward to the days we will remain pure in the Hagia Sophia Mosque. … May Hagia Sophia open, no matter why it is opened."

Orthodox Christians everywhere are watching. Visiting Hagia Sophia -- the museum -- has always created waves of "grief and awe at the same time," said Father Christopher Metropulos, founder of the Orthodox Christian Network on radio and the Internet.

"We know this will never be our cathedral again. That's not what this is all about," he said. "We all know that the status quo isn't perfect, but it beats the alternatives. … This is about losing access, once again, to a very important piece of our history."