Why were some churches and other older sanctuaries converted into mosques?

In the context of early Islamic history, the conversion of existing sanctuaries into mosques was not a universal practice, as Muslim conquerors often preferred to preserve local traditions or build new structures to meet the specific needs of their community.1

When older sanctuaries—such as Christian churches or Zoroastrian fire temples—were transformed into mosques, it was typically driven by a few key factors:

• Functional Necessity: The primary requirement for a mosque was a large, centralizing space for the community to gather for prayer and social affairs. In some instances, existing buildings were adapted because they could serve this purpose.1
• Symbolic Appropriation: In certain cases, such as the conversion of the Hagia Sophia following the conquest of Constantinople in 1453, the transformation served to establish the new faith's presence and authority in a conquered city. Similarly, the Dome of the Rock was interpreted as a victory monument asserting the religious claims of Islam over a holy city.23
• Islamization of Sacred Sites: Over time, a movement emerged to "Islamize" ancient holy places by associating them with Muslim prophets, heroes, or holy men, which sometimes involved repurposing existing sites.3

Despite these instances, many early mosques were built on previously unused land, particularly in newly founded cities like Kūfah and Basra, where the hypostyle mosque was developed to accommodate the growing community.1