The presidential decree was signed within hours of a court ruling Friday, which declared unlawful an 80-year old government decree which converted the building from a mosque into a museum.
Ayasofya Mosque, as it is known in Turkish, will now fall under the supervision of the government’s religious directorate.
The decree is the culmination of a long-held goal of Erdoğan, who has called for the building to be returned to the status of a mosque for years.
The court’s decision drew heavy criticism from the international community, as has the Turkish president’s stated aim of ending the building’s neutral usage.
The Greek culture minister, Lina Mendoni, released a statement condemning the decision, saying the court ruling "absolutely confirms that there is no independent justice" in Turkey, and that "the nationalism displayed by President Erdogan... takes his country back six centuries."
Eastern Orthodox Ecumenical Patriarch Bartholomew of Constantinople has said that the building’s prior status as a museum made it “the symbolic place of encounter, dialogue, solidarity and mutual understanding between Christianity and Islam.”
In a June 30 homily, Bartholomew said that Hagia Sophia, a UNESCO World Heritage site, belongs “belongs not only to those who own it at the moment, but to all humanity.”
Hagia Sophia was founded in 537 under the Emperor Justinian. For a time it was the largest building in the world and the largest Christian church. It served as the cathedral of the Patriarch of Constantinople before and after the Great Schism split Western and Eastern Christianity into the Catholic and Eastern Orthodox Churches.
After the Ottoman capture of Constantinople in 1453, the cathedral was converted into a mosque. Under the Ottomans, architects added minarets and buttresses to preserve the building, but the mosaics showing Christian imagery were whitewashed and covered.
In 1934, under the leadership of President Mustafa Kemal Atatürk, the founder of modern Turkey after the fall of the Ottoman empire, the mosque was turned into a museum.
The conversion of Hagia Sophia into a museum was considered a symbol of the Atatürk government’s commitment to building a secular liberal state. Mosaics were uncovered, including depictions of Christ, the Virgin Mary, John the Baptist, Justinian I, and the Byzantine Empress Zoë Porphyrogenita.
When the museum reopens for worship as a mosque, it is believed that the mosaics will have to be covered during Muslim prayers, as well as the seraph figures located in the high basilica dome.
Hagia Sophia is one of Turkey’s most recognizable landmarks and its most visited site, drawing more than 3.7 million visitors a year.
Erdoğan’s spokesman, Ibrahim Kalin, said that “Opening up Hagia Sophia to worship doesn't keep local or foreign tourists from visiting the site."