New Family Trouble…

There's not really anything you can do, other than to be a safe place for your future spouse to vent to. I would just focus on listening and empathizing with him. People who use religion as a weapon are not nice people. But that grandfather will have to decide for himself which is more important (his grandson or his religion); no one can force him to do the right thing.

Grandfather doesn't get to have control over how you live your lives. He sounds like a bit of a tyrant honestly. I would stop entirely dealing with that side of the family. You don't need their approval. I'm sad for your FI but this would have happened the second he went against them anyway, I'm betting. Now is a good time for him to stand up for himself and for your relationship. As Maggie said, all you can do is be there to support him.

Hello ! My husband is protestant and I am catholic. If someone had a problem with our protestant wedding, it would be my side of the family, but they didn't. Having a catholic ceremony for me it's like having a protestant one with benefits, as it's a real sacrament. Here in Europe all the protestants are so welcoming for other confessions, it's hard for me to imagine that they are not...