Probably the most well-known example of a church being converted into a mosque is the Hagia Sophia in Istanbul, Turkey. The building had been a Byzantine church from 360 until 1453, when the Ottomans repurposed it for a mosque after the conquest of Constantinople. It remained a mosque until the Ottoman Empire was dismantled after WWI. In 1934, Turkish President Ataturk turned it into a museum; in 2020, Turkish President Erdogan turned it back into a mosque. As Turkey's most popular tourist destination, it remains open to non-Muslim visitors.
Statements of condemnation for this act came from the Greek Orthodox Church, the Russian Orthodox Church, and the Roman Catholic Church. UNESCO, the U.N. organization that assigns world heritage status, expressed disappointment. However, this is not the first time the religious temple at this location changed hands. A pagan temple known as the Temple of Diana at Ephesus was on this site first. This temple was considered one of the seven wonders of the ancient world, and the site dates to the Bronze Age. This temple is even mentioned in the Bible:
"the temple of the great goddess Artemis (Diana) will be discredited; and the goddess herself, who is worshiped throughout the province of Asia and the world, will be robbed of her divine majesty." (NIV Acts 19:27)
The Christians took over a pagan site in 360, and eleven hundred years later, the Muslims took over the same location, which was then a Christian house of worship. Events like these are common throughout history.
The process for Muslims starts in 630. According to Islamic legend, the Ka'aba was a small cubic house of worship built in Mecca by Abraham and Ishmael. It became a polytheistic house of prayer with up to 350 idols. In 630, Muhammad led a pilgrimage of 10,000 warriors to Mecca, and the city converted to Islam. The Ka'aba was the first non-Muslim sanctuary to be reconverted into a Muslim house of prayer or a mosque, and Muhammad himself accomplished this. Other than this direct action by Muhammad, there is no explicit teaching in the holy book about the question of seizing or not seizing churches.
Islam expanded throughout the Middle East into India, Persia, Turkey, Africa, and even Spain for the next few centuries before meeting European resistance. According to early Muslim historians, the towns that surrendered without resistance and made treaties with the Muslims received permission to retain their houses of worship. In the cities that resisted, the Muslims seized places of worship. The cycle of life for a house of prayer had a familiar path. For example, in Spain, many churches were built on the sites of Roman temples because the foundations and building blocks were already in place. During the Reconquista, Christian warriors were just as often to appropriate and convert mosques to churches as the Muslims did to repurpose churches into mosques. The Great Mosque of Cordoba began life as a church, was converted to a mosque at the time of the Muslim conquest and reconverted to a church at the time of the Christian reconquest of Spain.
It's not only churches that have been converted. Some 2000 Hindu temples in India were confiscated during the Muslim expansion of India. I recently posted an article about a high official in the ruling party in India saying blasphemous things about Muhammad (see "India Faux Pas Causes Worldwide Boycott, June 25, 2022). What caused her emotional outbreak was that surveyors found the foundations of a famous Hindu temple under a mosque. The Hindus wanted to investigate further, and the Muslims refused.
After the Islamic conquest of Persia, Zoroastrian fire temples were usually turned into mosques. This act reveals a bit of irony. As the Islamic empire grew in the sixth century, they were casting about how to mint coinage. For their gold coins, they copied Byzantine designs on their coins. For their silver coins, they copied the fire pits from the Zoroastrian coinage. See the accompanying image of such a fire altar. In addition, Muslim armies converted synagogues and Sikh gurdwaras (temples) as opportunities presented.
Some of the more well-known sites that sport(ed) a mosque are:
The Dome of the Rock on the temple mount. A Byzantine church was there first.
The Parthenon in Athens. Again, it was preceded by a Byzantine church.
The Cave of the Patriarchs. It was built by Herod the Great and became a Byzantine church, a mosque, and then a synagogue.
The forced conversion of non-Islamic places of worship into mosques has waned in modern times, as no significant territorial acquisitions have recently been made. However, another phenomenon is taking place. Christianity membership is declining in Western Europe and the United States. Islam is filling the vacuum. In the U.K., The National Churches Trust reports more than 2000 church closures over the past decade – a period in which weekly church attendance decreased by a fifth. The U.S is no different. About 4000 churches a year close compared to about 1000 that start up. What do you do with all this real estate?
Another statistic to keep an eye on is the amount of zakat (money given to Muslim charities) worldwide exceeded $50 billion. Muslims are buying churches and private schools (Islamic Centers for Education) in each major city and media outlet centers to print books, magazines, and advertising. According to some security analysts, buying churches is one form of jihad for the sake of Allah. As the Muslim population expands, it is only natural for Muslims to place mosques where their population resides. In several cases, Christians have moved on, and Muslims have taken their place. It is only natural that they repurpose real estate for their specific needs, and that would include churches as well.
About the coin in the accompanying photo: The image presented is a Zoroastrian fire altar on a Sasanian (Persian) silver drachm dated to 438. The picture shows two attendants with swords on either side of the altar. The flame of the fire rises from the altar.
The Muslims used the Sasanian format for their silver coinage between 651 (A.H. 31) and 697 (A.H. 77). On the obverse, an image of the ruling caliph, and on the reverse, a picture of the fire alter with Arabic inscriptions encircling the coin on both the front and reverse sides.
Credit to Wikipedia Commons for the image of the Parthenon in Athens, Greece. While first being a temple to pagan gods, it became a Byzantine church and then was converted to an open sky mosque.