Is it permissible to attend an iftar feast with other Muslims in a mosque for the sake of interfaith in the community.
There are two questions here. First, if one can eat together with non Jews. Second, if it’s OK for a Jew to enter a mosque.
A Jew is not allowed to eat a festive meal with non Jews. Some apply this to all meals so long as the Jews and non Jews are sitting at the same table and the majority is non Jews. According to these opinions it applies even if the food is supplied by the Jew and hosted in the Jews home.
However, if the majority of people are Jewish or they are sitting at separate tables it is permissible, even if paid for by the non Jews or hosted in a non Jewish home.
Regarding the second question, one is only allowed to enter a mosque when there is a great need.
You mentioned “interfaith in the community”. That can be a problem in itself. This wasn’t addressed in the answer.
שו”ע יו”ד סי’ קנב, ופ”ח בסי’ קיד. ועיין בין ישראל לנכרי יו”ד פכ”ט ובס”ק יג. וראה בארוכה אצלנו שאלה 1978.
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