I don't think songs of worship belong in song circles, for the very reason you are having a problem. I see nothing wrong in kindly telling the whole group (so the offenders don't feel too singled out) that there is a policy, no religious songs, the group being secular and/or multi-faithed. Furthermore, altho I am not religious myself, I have respect for the faiths of others, and it seems disrespectful to me to sing religious songs as just part of a sing-along, without commitment or belief.

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