Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.
Wa alaikum assalam wa rahmatullahi wa barakatuh.
As to what follows:
Scholars are in consensus that if one performs the prayer in the church in a clean location then his prayer is permissible and valid. This was conveyed by Ibn `Abdul-Barr in his book al-Tamhid (5/229).
They differed, however, in regards to whether prayer is disliked in it or not. A group of scholars and jurists opined that prayer is disliked in a church because of what it contains of statues. This was reported from `Umar, Ibn `Abbas, Malik, and is the opinion of the Shafi`i Madh'hab.
Others have made a concession regarding entering it if there are no statues, such as Hasan Basri, `Umar bin `Abdul-`Aziz and Sha`bi, and it is the opinion of the Hanbali Madh'hab. If not, then prayer in it is disliked.
As for the Hanafi Madh'hab, some scholars have opined that it is disliked, whereas others opined that it is forbidden as it is a location for devils, and praying in it would be a means of venerating them.
What is apparent here is that if there are statues in it, then prayer inside of it is disliked. On the other hand, if there are no statues in it, then the prayer would be permissible, but one should not abandon praying in mosques and instead intend to pray in churches, as that is not permissible. It is obligatory for one to pray in mosques if they are found and one should not substitute them with anything else.
Your brother,
Dr. Khalid al-Muslih
28 / 2 / 1426 AH