Turkish President Recep Tayyip Erdoğan signed a decree on July 10 to re-convert the magnificent Hagia Sophia — the onetime cathedral of the Bynzantine Empire — in Istanbul into a mosque. The order was issued within hours of a court ruling that wiped away an 80-year-old government decree that had declared the beloved Christian symbol a museum. The July decision has been condemned by Christians all over the world, as well as international leaders and the United Nations.

Here are eight things to know about the Hagia Sophia, a UNESCO World Heritage site.

1. It was the cathedral of the Eastern Roman Empire for more than 1,000 years.

Hagia Sophia was constructed as the cathedral-church of Constantinople, and from 527 until Sept. 29, 1453, and the fall of the city to the Muslim armies of the Ottoman Empire, it was the largest and arguably greatest church in all of Christendom. The name of the church is translated as “the Holy Wisdom,” and it was dedicated to the Second Person of the Trinity.

2. There were several churches built on the site.

In fact, the Hagia Sophia was the third church built on the same site. The first church was built in 360, during the reign of Emperor Constantius II, son of Emperor Constantine the Great, the founder of Constantinople. Called at first the “Great Church,” it was destroyed by fire in 404, when the outraged supporters of St. John Chrysostom protested his exile by the Empress Aelia Eudoxia, who hated the saint and archbishop of Constantinople for his criticism of the corruption, opulence and brazen immorality of the imperial court.

The second church was built at the command of Emperor Theodosius II in 415. The massive five-aisled church was also destroyed, this time by the bloody Nika Revolt — a violent protest against Emperor Justinian I in 532 that was brutally suppressed but that also ended with the destruction of much of the capital. As reconstruction was essential for the church and whole parts of the city, Justinian decided to embark on a truly ambitious project that would create an unprecedented masterpiece of a cathedral. The result was the Hagia Sophia.

3. It is honored as one of the greatest masterpieces of art and architecture.

Building the Hagia Sophia took five years, from 532 to 537, and involved thousands of artisans and laborers. The chief architects for the church were two of the greatest minds of the era: Isidore of Miletus, a physicist and mathematician, and Anthemius of Tralles, a geometrician and mathematician.

The plan included a rectangular basilica with an immense dome — with a magnificent cross at the very top — with materials from all over the Byzantine Empire, including bronze, marble, ivory, gold and stone from Egypt, Syria and Greece. Columns from the onetime pagan Temple of Artemis in Ephesus were also hauled to Constantinople for use. The dome was revolutionary in its design, so much so that after an earthquake in 557 it collapsed partially the next year. The construction of a new dome, with greater architectural supports, or ribs, was overseen by Isidore the Younger, nephew of Isidore of Miletus.

The interior was also extraordinary in its opulence and its interplay of light and shadow, thanks to the ring of windows seemingly floating beneath the main dome. The decorations in the church eventually included mosaics depicting Christ, the Blessed Mother and angels. A Christ Pantocrator dominated the central dome. One of the other important floor decorations was in the nave — the Omphalion, a slab of marble used for the coronation of the Byzantine emperors.

It was officially dedicated on Dec. 27, 537, and Emperor Justinian supposedly exclaimed when it was finished, “Solomon, I have outdone you!” The final dimensions of the church: 269 feet long, 240 feet wide; the cupola of the dome soars 180 feet above the main floor. It remained the largest cathedral in the world for almost 1,000 years and the zenith of Byzantine architecture.

The sixth-century historian Procopius of Caesarea wrote that the Hagia Sophia was “distinguished by indescribable beauty, excelling both in its size, and in the harmony of its measures, having no part excessive and none deficient; being more magnificent than ordinary buildings, and much more elegant than those which are not of so just a proportion. The church is singularly full of light and sunshine; you would declare that the place is not lighted by the sun from without, but that the rays are produced within itself, such an abundance of light is poured into this church.”

4. It survived earthquakes, fires and Iconoclasts.

Aside from the earthquake of 557, the Hagia Sophia survived several natural disasters. A fire in 859 was followed a decade later by an earthquake. Another quake in 989 caused damage to the dome, and Emperor Basil II commissioned a famed Armenian architect, Trdat, to oversee the repairs. Earthquakes have occurred over the centuries, including one in 1894.

The interior was also impacted in the eight century by the Iconoclast movement under Emperor Leo III the Isaurian, who issued a decree forbidding images in the empire. The mosaics in the Hagia Sophia were either removed or covered over. The decree began two periods (from around 726-787 and from 814-843) of Iconoclast influence. When, however, the ban on images ended, Byzantine Emperors installed new and beautiful images of Christ, the Blessed Mother and saints.  

5. It was sacked in 1204 by Crusaders during the Fourth Crusade.

During the Fourth Crusade in 1204, the Hagia Sophia was sacked by Latin Crusaders when they seized Constantinople and overthrew the reigning Byzantine imperial government. The dark moment in Christian history was orchestrated by the cunning 90-year-old blind Doge of Venice, Enrico Dandolo. The church was looted, with many sacred vessels and relics taken back to Italy, and the Hagia Sophia served as a Latin church until 1261, when the Byzantine imperial line reestablished itself. When Dandolo died in 1205, he was buried in the Hagia Sophia. The events of 1204 continue to cast a shadow on efforts to heal the Great Schism of 1054 between the Orthodox and Catholics.

6. It was forcibly converted into a mosque in 1453.

Muslim armies tried for centuries to conquer the Byzantine Empire. The final defeat of the Byzantines came at the hands of the Ottoman Turks, who systematically destroyed the collapsing empire until only Constantinople remained. The great city finally fell on May 29, 1453, and the last emperor, Constantine XI Palaeologus, died defending the city gates. The Ottoman army under Sultan Mehmet II poured into the city, and Ottoman troops were allowed three days to rape and slaughter the inhabitants and loot the city. The survivors were placed into slavery if they could not be ransomed. 

The thousands of Byzantines who fled to the Hagia Sophia to pray for help suffered the same fate when frenzied Ottoman soldiers stormed the cathedral. Like the other churches in the city, the Hagia Sophia was sacked and desecrated. By the time the sultan finally entered the church, it had suffered significant damage.

Mehmet declared that the Hagia Sophia would henceforth serve as the main mosque of his new capital. He ordered the mosaics to be covered in plaster and then decorated with Islamic calligraphy and designs. The sultan completed the forced conversion of the church into a mosque with the installation of a minbar (pulpit), mihrab (prayer niche) and fountain for washing to be added. Over the next centuries, four minarets for the call to prayer were added at the four corners of the building.

Aside from the images being covered over, the most significant change to the interior was the installation of eight huge medallions on the columns in the nave. They were added during extensive renovations between 1847 and 1849 under Sultan Abdülmecid I and display the names of Allah and Muhammad, the first four caliphs Abu Bakr, Umar, Uthman and Ali, and Muhammad’s grandchildren Hassan and Hussein. The restoration was overseen by the Swiss-Italian architect-brothers Gaspare and Giuseppe Fossati and a crew of almost 1,000 workers.

7. It was declared a museum in 1934.

After the abolition of the Ottoman Sultanate in 1922 and the start of the Republic of Turkey, a trend toward secularization began under its first president, Mustafa Kemal Atatürk. The Hagia Sophia was closed in 1931, and in 1934, Atatürk officially declared it a museum. As part of the new status, the mosaics were publicly revealed for the first time in centuries. In 1985, the Hagia Sophia was included in the “Historic Areas of Istanbul” and therefore part of UNESCO’s World Heritage List. It was also one of Turkey’s most-visited sites, with several million visitors every year.

8. World Leaders have condemned declaring the Hagia Sophia a mosque.

World reaction to Erdoğan’s July 10 decree to revert the Hagia Sophia to a mosque has been almost universally negative.

The director-general of UNESCO, Audrey Azoulay, wrote, “Hagia Sophia is an architectural masterpiece and a unique testimony to interactions between Europe and Asia over the centuries. Its status as a museum reflects the universal nature of its heritage, and makes it a powerful symbol for dialogue.”

The Eastern Orthodox Ecumenical Patriarch Bartholomew of Constantinople, whose cathedral would historically have been the Hagia Sophia, expressed his great dismay, declaring in a homily on June 30, “The conversion of Hagia Sophia into a mosque will disappoint millions of Christians around the world, and Hagia Sophia, which, due to its sacredness, is a vital center where East is embraced with the West, will fracture these two worlds, more so at a time when the afflicted and suffered mankind, due to the deadly pandemic of the new coronavirus, is in need of unity and common orientation.”

Matthew Bunson is executive editor and Washington bureau chief of EWTN News.