Hagia Sophia was built in 537 under Emperor Justinian and served as the cathedral of the Patriarch of Constantinople. After the Ottomans captured the city in 1453, the cathedral was converted into a mosque.
The building was turned into a museum under Mustafa Kemal Atatürk, the first president of the secular Republic of Turkey, which was established in 1923 following the collapse of the Ottoman Empire.
Bo said that his concern over the status of the UNESCO World Heritage Site was part of his wider desire to uphold religious liberty around the world.
"In my country, Myanmar, mosques have been razed to the ground and I have spoken out -- frequently and at some risk. In China, the Uyghur Muslims are facing what amounts to some of the contemporary world's worst mass atrocities and I urge the international community to investigate. In India and Sri Lanka Muslims have faced appalling violence and I have condemned such inhumanity," he said.
"Similarly, In Indonesia, Ahmadiyya Muslim mosques have been destroyed by other Muslims, and churches have been forcibly closed. In Iran the Baha'is face an intense assault on their freedoms, and in Syria and Iraq sacred places have been wantonly destroyed while, sadly, closer to home, we have seen the same phenomenon in China with shrines destroyed, the Cross removed from places of worship, and even churches, like Xiangbaishu Church in Yixing, demolished."
"Turning Hagia Sophia into a mosque represents a similar undermining of freedom of religion or belief, love for each other, respect for the dignity of difference."
When Hagia Sophia reopened as the Ayasofya Mosque July 24, Erdogan and other senior officials joined hundreds of worshipers inside the building for Friday prayers, while large crowds filled the streets outside.
The U.S. Conference of Catholic Bishops has backed an initiative by the Greek Orthodox Archdiocese of America to observe July 24 as a "Day of Mourning."
July 24 is the anniversary of the 1923 Treaty of Lausanne, which established the borders of the modern Turkish state and included explicit protections for Christian minorities.
"At a time when humanity is enduring intense strains due to the global pandemic, we need to come together, not drive communities apart. We need to put aside identity politics, abandon power plays, prevent ethnic and religious conflicts and value the dignity of difference among every human being. And we must cherish diversity and the unity we find within it," Bo said.