If one sings in a secular choir (consisting of both Jews and gentiles) that performs in secular locations and contexts (such as concert halls, public plazas, nursing homes, etc), what are the issues if the choir selectively performs Christian music? I think singing that material could be avodah zara and at least has an issue of marit ayin; others I've spoken with think the performance context negates those issues, holding that it's about the music, not the sentiment, and choir members aren't assumed to agree with everything they sing. I would appreciate sources for either position (and others that people might raise).
For purposes of this question let's assume that we understand what "Christian music" is. (That may be a followup question later.) This question is about the circumstances (if any) under which it is acceptable to participate in a performance of theologically-objectionable music.