I'm not sure I agree with the pagan/atheist part, but I would suspect that many/most poly folk are not actively Christian. A practicing Christian would, by definition, be against poly in any form as a sin against God and man.
Personally I grew up Catholic/Baptist (yeah, I know) but I don't really believe in the Christian God/Trinity. I am not entirely sure I believe in any god or gods for that matter. I think there are things out there that are "more" than we are as humans ... but whether or not it's something divine is open to interpretation, I think.