See question above. Regarding Islam, however, most Poskim follow the opinion of the Rambam (Hilchos Ma’achalos Assuros 11: 7) that it is not considered Avodah Zarah (Shulchan Aruch Y. D. 124: 6, Taz 4 and Shach 12. See also Ben Ish Chai, Parashas Balak) Therefore, they do not expressly forbid entering a mosque. (Avnei Yashfei 1: 153 quotes HoRav Y.S. Elyashiv Zt’l as ruling that it is not prohibited to enter a mosque.)

Other more contemporary Poskim prohibit entering a mosque as well (Tzitz Eliezer 14: 91; 18: 47). See also Meiri, Avodah Zarah 57a, who quotes Chachmei Sefarad as ruling that Islam is Avodah Zarah. All Poskim agree that unless there is a compelling reason to do so, one should not enter. See question 1182 in regard to davening in a multi faith room at an airport, we wrote: “Beis Avi (4: 31), Shevet Hakehosi (6: 83), Menuchas Emes (3: 6: n.24) rule not to pray in a multi-faith space, so is it amply quoted in the name of Rabbi J. B. Soloveitchic zt’l. (See also Remoh O.H. 94: 9, on davening in a non-Jewish hotel). Horav Shlomo Miller’s Shlit’a opinion is similar, he suggests to daven outside of the non-denominational room. Care should be taken as an array of diverse faith symbols may be exposed at the entrance of the room.”