Breathe
Hostis humani generis
I completely agree.Since your in-laws are your in-laws for as long as the marriage lasts, I think it's best to try to iron out any issues beforehand. Hopefully, they will try to understand. It would be tough to put a wedge in the family.
Although to be honest, I imagine that any 'faith' issues can easily be overcome by discussion, for the most part, or are being used as a cover-up for the fact that the in-laws simply don't like "you" (not you personally ) and don't think their child should marry "you", or is related to issues of upbringing children.