I am staying with my family who is not religious and their good friends asked if we would help them set up their church rummage sale (in the basement of the church). I briefly mentioned that Jews are not really supposed to go into churches (I was saying it about me, not telling them not to go) but it didn’t go over so well. Are there any more lenient opinions on this — at this point I feel like if I don’t go it would be a chilul Ha-shem, it would make both my Jewish family and their non-Jewish friends have a negative view of Orthodox Judaism. What to do?!
It is not permitted to enter the Church.
Although the main prohibition refers to the area where prayers are recited, and not to general Church property, there are also a number of halachic instructions to distance oneself from a Church: See Rema Yoreh De’ah 149:2; Iggros Moshe 3:129; Shulchan Aruch Yoreh De’ah 150 and Darchei Teshuva 150:2; Chai Ha’Levi 4:63.
With regard to entering the basement of a Church for purposes of voting, see Teshuvos V’hanhugos 2:410 and Chai Ha’Levi 4:63, who both write that one can enter the basement for purposes of voting, providing there is nowhere else to vote. However, the case of voting is special, because it is known to all that the place is used for voting, as the poskim mention.
Note that many poskim write with great stringency about entering a Church, and Rashba expresses doubt even concerning entering in cases of pikuach nefesh (under such circumstances, however, Rosh explicitly permits entering). For a leading Sefardi ruling, see (at length) Yabia Omer, vol, 2, Yoreh De’ah no. 11. See also: Birchei Yosef 149:2, Darchei Teshuva 150:2, Igros Moshe 3:129, Minchas Elazar 1:53-3, Yechaveh Da’as 4:45, Binyan Tziyon 1:63, Shearim Metzuyanim B’Halacha 167:9, Rivevos Ephraim 3:496-497.
It is noteworthy that although the nature of many Churches has changed significantly from the times of Chazal, the general attitude of poskim to entering Churches has not changed significantly.
Therefore, you should try to get out of going into the Church, using whatever pretext you can. For entering the basement alone, there is room for leniency if the circumstances are truly extenuating, but this should be avoided if possible.