Conversion from churches

Many mosques of the first centuries, were originally churches. When churches were converted into mosques, this was naturally against the will of the Christians, but this wasn't always a big problem. In many regions, Christianity had lost its position, so the churches were simply turned into mosques over time. Muslims could actually use the churches since they were religious buildings and since Christianity was considered as a kin religion to Islam. This typically meant that both Christianity and Christians were regarded with respect by Muslims. Over time, many Christians gradually converted to Islam, and eventually, the building's use was more appropriate as a mosque than as a church.
Most mosques today are closed to non-Muslims, but this was a regulation that was developed in the first century of Islam. There was an increase in the emphasis on the sanctity of the mosque, more and more elements of the mosque were regarded as sacred, and any mosque was commonly regarded as "bayt allah," the "House of God."