My sifu is a Christian and he talks about it sometimes but he never preaches about it. My older sifu from American Kenpo never spoke of his faith or lack of it, I have no clue what his faith was.
One thing that bugged me about one of the tournaments we went to which was hosted by Marc Unger held had a few minutes of prayer before the tournament began. We were not forced to pray but some individuals would give us dirty looks if we did not. Sifu was one of those who did not pray and one man told him "you are sitting a poor example for your students." He said to him " we are here for kumite and not for cathecism."
I agree with him entirely. I don't mind a moment of silence to pray, I just don't like when you host it as something optional but negatively judge people for excersicing the choice to not do it.
If that's the case then only invite Christians to your tournament if you have such an issue with non Christians, or people who simply don't pray before the kumite.
I'd be pretty uncomfortable going to a martial arts event that had prayer in it too. I don't have any problem with people praying before they train or compete, but when you have an official group prayer as part of your event, even if you say it's optional, it can lead to the kind of uncomfortable situation you're talking about.