Shaykh ‘Abdullāh al-Bukhārī
(The Questioner) is asking about the ruling on the conversion of churches to masājid
The answer is that, it is permissible. (Because) it has been done (before).
I say: It has been done (converted) during the time of the righteous predecessors (from amongst the companions [may Allāh be pleased with them])
And this (converted Masjid) became a place where they would worship Allāh (mighty is His name) and they removed whatever signs of shrik and anything (related to shrik) from it.
And the Adhan (call to prayer) was established and it would not be evident that they used to worship other than Allāh (in that place) because the Adhan (call to prayer) is being established and symbols of shirk have been removed (from this place).
Yes. And (because) the complusory daily prayers are being observed congregationally within it and also the Jumu’ah prayers (are being observed as well) and other than that.
So, the defect was removed from it
(i.e the converted masjid).