Dear Rabbi Simon,
Please can you tell me if one may go to an exercise class in a church hall? If so, would that still be the case even if there’s a cross (crucifix) on the wall?
As a rule, entering the sanctuary of a church is to be strictly avoided. (I will not dwell on those occasions where Chief Rabbis and others have done so, but suffice to say the matter is halakhically complex, if not doubtful.) Nevertheless, the posekim have allowed entering a church hall (as in your question) for specific circumstances. An example of this is where the church hall serves as a voting station. This has the further advantage in that it is apparent to all that many people are entering the church building on this occasion for a particular (civic rather than religious) purpose. Some also allow one to attend an AA meeting or similar in such a location. By extension, a Weight Watchers meeting may also be allowed (not that such a purpose is of any relevance to you).
However, Rav Moshe Feinstein (Iggerot Moshe OH 4:40:26) explicitly forbids children from playing basketball in a church gym, partly for concern re influence or even missionary efforts which may be present there.
In the case of your question, I would take the more permissive approach. While a class in another location wouldbepreferable, if this is the one that suits your needs you may attend “with a clear conscience”.
If the room is not designated for prayer, the cross may be correctly regarded as an identifying symbol rather than an object of worship. So yes, it is still permissible.
Rabbi Rashi Simon