Something similar happened to me about 15 years ago....I think it was an honest case of mistaken assumptions all round. Their rep was as a Christian band and their advertising had asked for a Christian bassist, but was new to the area (Virginia/Maryland/DC) and hadn't heard of them, and was referred to the gig via word of mouth, so they thought I knew what they were about, and the guitarists had Marshalls and Les Pauls, which are not things I had associated with Christian acts at the time. Rock and roll.
Two rehearsals, then first show, halfway through the first set they took a break between songs to introduce me as the new bass player and ask me to witness to the audience.
Er, what?
I'm not an evangelical Christian, and at that point I honestly had NO idea what the term 'to witness' meant, so I was standing there with a dumb expression on my face and no clue what was going on.... and I could see this really puzzled look on the singer's face... so I said FREE TIBET! and that got a cheer and we started playing again before the audience realized the wheels had come off.
My involvement with the band ended that night with some red faces but no hard feelings. I never had any pressure from them, but I just don't wear religion on my sleeve like that.