Via
the Christian Science Monitor:
The ACLU, quite obviously not some conservative partisan outlet, is supporting a girl who was banned from singing the song "Awesome God" at a school talent show, claiming that the song was "too graphic and violent, and crossed the line into proselytism."
The case involves protected student speech rather than government-endorsed speech, agrees the ACLU's Edward Barocas. "This was not a mandatory assignment. This took place at an after-school event that was voluntary where the individual student could decide what song to sing or what skit to perform," Mr. Barocas says. "It would be a different analysis if the principal sang the song 'Awesome God' over the loudspeaker at school."
I think that this quote gives a sense of when religious expression is acceptable in the public sphere. Authority figures can't endorse a particular religion. Those exposed to it can reasonably avoid it, not showing up to an event or walking around an object.