The mishna on 47 B states that if a house housed avoda zarah, once the avoda zarah is removed, the house becomes permitted.
Is this permission only for a Hedyot, regular use, like the gemarah's conclusion on 46 B . But for Machsheie Korban, Gavoah, i.e. in our case, a shul, it would be forbidden, or is there no difference ? The difference would be if a church was for sale, it could definitely be converted for private use, a house. But how about using it for a Shul ?
This subject was discussed in detail in the Yosef Da'as for Daf 46, which you can find on our website (http://www.dafyomi.co.il/azarah/yosefdas/avod-mnue.htm - copy of pertinent part attached).
In short, the Mishnah Berurah (154:45) sums it up that the MAGEN AVRAHAM permits making a church into a Shul, the ELIYAH RABA says that it should not be made into a permanent place of Tefilah, and the BI'UR HALACHAH adds that all opinions would prohibit making a house of worship into a Shul if an idol was actually located in the house of worship. (That is, the church under discussion is one in which no idols are located, but in which other religions simply pray).
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