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.