Many observers see the controversial move as part of Erdogan's bigger attempt to reenact the Ottoman Empire expansionism in the Middle East by pushing his Islamist agenda.
"I was very much shocked by the news that Hagia Sophia had become a mosque. It's a provocative act," Jesuit Father Samir Khalil Samir, a noted Egyptian Catholic theologian and Islamic studies scholar, told Catholic News Service.
"This monument from the sixth century belongs to the whole world. It is in Turkey, but it belongs to those who built it, the Christians, and for a time Islam took it. I'm sure the decision will play against Erdogan, even in the Muslim world," said Father Samir, who founded the Arab Christian Documentation and Research Center in Beirut and taught at the Pontifical Oriental Institute in Rome.
An architectural masterpiece, the massive Hagia Sophia was built as an Orthodox Christian cathedral and stood as the seat of Eastern Christianity for a thousand years before Ottoman Turks conquered its host city, then known as Constantinople, in 1453. Thousands of Christian Armenians, Assyrians and Greeks were persecuted at the time. Ottoman Turks turned the cathedral into a mosque and, in 1934, it was designated as a museum by modern Turkey's founding statesman, Mustafa Kemal Ataturk, a secularist.
"What can I say as a Christian clergyman and the Greek patriarch in Istanbul? Instead of uniting, a 1,500-year-old heritage is dividing us. I am saddened and shaken," Ecumenical Patriarch Bartholomew of Constantinople told The Washington Post of Erdogan's action.
The spiritual leader of approximately 300 million Orthodox Christians worldwide reaffirmed that in Istanbul "we have survived for 17 centuries and we will stay here forever, as God wills."
Greece has threatened sanctions against Turkey, while the United States and other Western nations had earlier urged Erdogan to maintain the iconic structure as a museum in keeping with Istanbul's multireligious heritage and its status as a symbol of Christian and Muslim unity.
Pope Francis said he was "very pained," by Erdogan's declaration renaming "The Grand Hagia Sophia Mosque," while the World Council of Churches expressed "grief and dismay."
"This move confirms the inseparability of political and religious sphere of Islam," noted a Catholic priest who has served for several years in Turkey, but who wished to have his name withheld.
"The big deal is to repeat, or reinvent what happened at the conquest of Constantinople; the victory of Islam over every culture and religion existing before it. It certainly concerns the past and it might concern the future," he told CNS. "It means once again that the leadership of Turkey is choosing the way of political Islam, which is irreconcilable with any other political reality."
Dominican Father Alberto Fabio Ambrosio, a specialist in the history of Ottoman Sufism, agrees, saying that "in this perspective, it impacts the relation between religions, the interreligious dialogue."
But Father Ambrosio, a professor of theology and history of religions at Luxembourg School of Religion and Society, adds that Erdogan, an avowed Islamist, is playing to his own populace, "the most Islamic traditionalist part of the country. He is saying to Turkey and to the entire world that a part of the Kemalism culture or ideology (Ataturk's secularism) is definitely dead."
"He is telling the Kemalists: Your interlude was a parenthesis," Elizabeth Prodromou told the Wall St. Journal. Prodromou, who works with the Center for Religious Freedom at the Hudson Institute in Washington, said Erdogan envisions Turkey displacing Saudi Arabia as leader of the Sunni Muslim religious world.
"This decision shows the idea of pan-Ottomanism, or the New Ottomanism project begun by Erdogan and former Prime Minister (Ahmet) Davutoglu," Father Ambrosio told CNS. But Davutoglu has mounted his own criticism of Erdogan, accusing him of failing to manage Turkey's faltering economy and corruption. He said Erdogan has failed to disclose the whereabouts of 110 billion Turkish liras (more than $16 billion).
Turkey's internal problems may also explain Erdogan's adventurism abroad, entangling its military in Syria's longstanding conflict by employing Islamist militants such as the Islamic State and al-Qaida to fight Kurds, Christians and Yazidis there. Erdogan is also sending Turkish and Syrian fighters to Libya and Yemen, in a bid to resurrect Ottoman Turkish dominance over Arab lands, but Father Samir says the Arabs want none of it.
"He would like to create a new caliphate," said Father Samir. "Erdogan is someone who pretends to be the greatest 'king' in the Middle East, while at the same time provoking a lot of Arabs and Muslims against him."
Earlier this year, the Turkish president tricked Syrian refugees to go to Turkey's northwestern border to try to storm into Greece. Erdogan has repeatedly threatened to "flood" Europe with refugees and foreign migrants as in 2015, while forcibly returning Syrian refugees back home in violation of the U.N. refugee agency protocols against forcible repatriations.
Meanwhile, Christians and Muslims alike are asking why Hagia Sophia cannot be used by both communities.
"Couldn't this magnificent church reflect its 900 years of Christian and 500 years of Islamic history by letting Muslims and Christians pray inside it?" asked Thomas Sternberg, president of the Central Committee of German Catholics, a laypeoples' organization.
"This could be a large and unique sign of mutual respect and a gesture of deep religious understanding," said Aiman Mazyek, chairman of the Central Council of Muslims in Germany, which also includes members of Germany's Turkish and Arab communities.