The phrase "to convert the church" means to change or transform the beliefs and practices of a religious institution, usually with the goal of making it more aligned with a particular ideology or teachings. Full definition
Any general attempt to convert church groups into therapy would produce widespread (and justified) resentment and would miss the broader educational, prophetic, and service goals of a church. (religion-online.org)
The hostel operates in a lively converted church with Gothic vibes. (edinburghhostels.com)