Q. Can you daven lekatchila in a shul that was once a Catholic church?

A. Mishna Berura (154: 45) quotes Mogen Avrohom that if the building has been sold and the idols and statues were removed it is permitted, since the building itself was not worshiped. However, in Biur Halocho he mentions that when idols that are worshiped were placed in the building (as is usual in Catholic churches), although the building after being sold and the idols removed is now permitted for common use, it should not be used as a shul. Many Poskim support this view (Eliyahu Rabba (154:15), Pri Megodim (Eshel Avrohom ibid. 17), Chassam Sofer (Orach Chaim 42), Bnei Zion (63),Maharam Schik (Y. D. 154) et. al.)
Igrois Moishe (O. H. 1: 49) writes that although it became customary in our countries to convert churches into shuls, relying on the opinions of Mogen Avrohom and Mishna Berura, he does not support the view and considers this to be “mous” or repugnant. However, after the fact, he agrees that it is permitted. He recommends to totally remodel the inside of the walls, so the location acquires a new face or ponim chadoshos.
Various denominations of Christian churches do not maintain in their temples any idols that they actually worship and would therefore be permitted when sold and the symbols of their religion are removed.
Chashukei Chemed (Avoda Zarah 47a) mentions that Horav Eliashuv’s zt’l opinion was to be lenient following view of the Mogen Avrohom and Mishna Berura when the inside is redone.
He also permits entering the former church to examine and determine the changes needed.
Horav Shlomo Miller’s Shlit’a opinion is also to be lenient when remodeling is done. He additionally recommends to use the property for another use a complete year, before turning it into a shul.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a