In a previous life I taught world religions. I learned many lessons the hard way (but have forgotten most of them by now). Some things I do remember learning
1) there should be ground rules
2) those ground rules should presented as clearly as possible as early as possible.

Thus my syllabi were passed out on day 1 and contained something like the following:
"Please feel free to share your own experiences as a participant in a religious tradition. However, please do NOT witness for your faith. This class is NOT an appropriate place to proselytize for your faith. It also NOT an appropriate place to criticize faiths other than your own."
Having passed out the syllabus, I then read this particular passage aloud.

Seemed to work.