I apparently feel exactly opposite most people commenting on this do.
If he doesn't want a poly relationship, then that's his prerogative. If he still has those Christian beliefs, then asking him to let you be with other people could cause some major psychological issues for him.
I think making a conscious decision to lie and cheat to your spouse is a terribly hurtful thing to do. Cheating when you get caught up in an unexpected passionate moment is one thing. Purposefully seeking after careful consideration is another. I personally have a zero tolerance policy on cheating, either way. Always have.
I think you are both doing things that are hurting the other (him by not accepting your poly-ness and you by continuing to disrespect your relationship with him) and it needs to stop.
Which is more important? Him or poly? That is the real question. It doesn't matter what happens after, it doesn't matter what successes or failures you read about, what YOU want is what matters.
You've said he's tried threesomes, you've said he's read a bit even if he hasn't done a lot of research, so he has made some effort before saying there's no way he wants this. Now it's your turn to officially decide.