I would be extremely upset and would have difficulty coming to trust the owners, administrators, and any imams working there. I have been to a number of Mosques/Masjids, talked with even more Muslims of various sects. None of them have hidden their Islam, and none have pretended to be a “church” before or after. Many times, they have bought a church, converted it into a mosque, later sold it to another [Christian] Church. Mosques open and close, like many Christian congregations’ churches - moving into a building, then moving out again. In one case, the place was a called an “Islamic Foundation”, which did not seem wrong to me. “Islamic” made it clear it was not a “church” or “synagogue” or “temple.” It was also a mosque, obviously, but they chose to give it this name. They had all kinds of activities there. Likewise a Mormon Stake Center is called a Stake Center, not a “church” even though it obviously is a sort of church, and a “cathedral,” too, is called a “cathedral” although it obviously is a sort of church. To say you are building a “church” and then build a mosque, is like announcing you are building a new mosque in Mecca and then build a Roman Catholic Cathedral. People who would do that are less trustworthy than those who are up-front about who they are and what they are doing.