Catholic bishops across the United States have issued joint statements with their Greek Orthodox counterparts expressing sorrow at the reopening of Hagia Sophia as a mosque on Friday. 

July 24, was declared a "Day of Mourning" as the former Byzantine cathedral opened for formal Islamic Friday prayers for the first time in more than 80 years on July 24. 

Hagia Sophia had been a museum since Turkey's establishment as a secular state. Turkish president Recep Tayyip Erdoğan signed a decree July 10 converting it into a mosque following a ruling by the Council of State, Turkey's highest administrative court, earlier that day which declared unlawful an 80-year old government decree converting the building from a mosque into a museum.

Religious leaders around the world, including Pope Francis, decried the move, with the pope saying it caused him "great sadness."

On Tuesday, the U.S. Conference of Catholic Bishops said that Friday would be observed as a "Day of Mourning" and that Catholics would join the Greek Orthodox Archdiocese of America "in offering our prayers for the restoration of Hagia Sophia as a place of prayer and reflection for all peoples."

On Friday, Cardinal Blase Cupich of Chicago and Greek Orthodox Metropolitan Nathanel of Chicago released a joint statement saying they were "troubled by the government's disregard for religious pluralism in Turkey." 

"We are particularly concerned because this action represents a visible marginalization and continued attack on the religious freedom of Turkey's Christian communities and other religious minorities," the cardinal and metropolitan said. 

"Together, we join all those who mourn this divisive act and urge the world to stay vigilant in protecting religious minorities and religious freedom in Turkey." 

In Boston, Metropolitan Methodios and Cardinal Seán O'Malley issued similarly critical of the change. 

The conversion of the building into a mosque "alters the status quo that has existed for the last 85 years and causes great pain to many throughout the world," said O'Malley and Metropolitan Methodios. 

The two pointed out that the Hagia Sophia, which was completed in the year 537 before its forced conversion into a mosque, following the capture of Constantinople in 1453, had "served as a preeminent place of Christian worship for almost twice as long as it did a mosque." 

"Since its conversion into a museum, countless visitors have passed through its great bronze doors and appreciated its distinctive architecture and historic mosaics," they said. "It is as much an engineering marvel today as it was when the Eastern Roman Empire built it. This is truly a unique building that the entire world admires and respects." 

O'Malley and Metropolitan Methodios urged Erdogan to restore the building once again to a museum, and said that doing so would strengthen all religious communities in Turkey. 

Archbishop Gregory Hartmayer, OFM Conv. of Atlanta and Metropolitan Alexios of Atlanta released their own joint statement on July 20. 

"This current situation grieves us as both Christian Hierarchs, and as citizens of this great land whose commitment to religious tolerance still shines forth as a beacon to other nations,"they said. 

"The fact that the Turkish government would choose to reverse this decision of nearly a century, violates not only that same spirit of tolerance, but also insults the faith and wounds the hearts of Christians worldwide, Orthodox and Catholic alike. 

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Hartmayer and Metropolitan Alexios requested that Christians throughout the country "not only pray for, but speak up for Hagia Sophia."

"We must all do our part, through whatever means at our disposal, to ask that our elected officials pressure the Turkish government to reaffirm its commitment to the principles of religious tolerance and mutual respect," they said. 

While religious leaders condemned the decision, the organization In Defense of Christians, along with the Hellenic American Leadership Council and the Armenian National Committee of America, called for a boycott of Turkish products and services in response to the conversion of Hagia Sophia into a mosque. 

"Christians in the Middle East have faced genocide, destruction of their homelands, and persecution from Turkey. Enough is enough," said a statement from In Defense of Christians President Toufic Baaklini.