Its a major sin converting the church into a mosque. Even in the holy quran Allah swt orders the Muslims to protect the Churches!
Were Allah not to check people by means of others, there would have been demolished the monasteries, churches, synagogues, and mosques in which the name of Allah is often mentioned.
Surat Al-Hajj 22:40
This is the treaty of Prophet Muhamed with the ortodox Christians of the Monestery of St. Catherine Mount Sinai
It was about this time [after the Treaty of Hudaybiyyah] that the Prophet granted to the monks of the Monastery of St. Catherine, near Mount Sinai, his liberal charter by which they secured for the Christians noble and generous privileges and immunities. He undertook himself and enjoined his followers, to protect the Christians, to defend their churches and the residences of their priests and to guard them from all injuries. They were not to be unfairly taxed; no bishop was to be driven out of his diocese; nor Christian was to be forced to reject his religion; no monk was to be expelled from his Monastery; no pilgrim was to be stopped from his pilgrimage; nor were the Christian churches to be pulled down for the sake of building mosques or houses for the Muslims. Christian women married to Muslims were to enjoy their own religion and not to be subjected to compulsion or annoyance of any kind. If the Christians should stand in need of assistance for the repair of their churches or monasteries, or any other matter pertaining to their religion, the Muslims were to assist them. This was not to be considered as supporting their religion, but as simply rendering them assistance in special circumstances. Should the Muslims be engaged in hostilities with outside Christians, no Christian resident among the Muslims should be treated with contempt on account of his creed. The Prophet declared that any Muslim violating any clause of the charter should be regarded as a transgressor of Allah’s commandments, a violator of His testament and neglectful of His faith. (np)