This is because “Allahu akbar” does not mean “Allah is great,” as AP has it here, but “Allah is greater,” i.e., superior to your god. The conversion of Hagia Sophia from a museum to a mosque is a simultaneous declaration that Islam is superior to both secularism and Christianity. It is a manifestation of Erdogan’s ever clearer determination to cast himself as a new Mehmet the Conqueror and to restore the Ottoman caliphate.
“Turkey’s President Recep Tayyip Erdogan formally makes Hagia Sophia a mosque,” by Suzan Fraser, Associated Press, July 10, 2020:
ANKARA, Turkey — The president of Turkey on Friday formally converted Istanbul’s sixth-century Hagia Sophia back into a mosque and declared it open for Muslim worship, hours after a high court annulled a 1934 decision that had made the religious landmark a museum.
The decision sparked deep dismay among Orthodox Christians. Originally a cathedral, Hagia Sophia was turned into a mosque after Istanbul’s conquest by the Ottoman Empire but had been a museum for the last 86 years, drawing millions of tourists annually.
There was jubilation outside the terracotta-hued structure with its cascading domes and four minarets. Dozens of people awaiting the court’s ruling chanted “Allah is great!” when the news broke. A large crowd later prayed outside it.
In the capital of Ankara, legislators stood and applauded as the decision was read in Parliament…