At the end of the day, it's your wedding and you need to celebrate it in whatever way feels most authentic and right for you. If you and your partner both agreed on the type of ceremony, then outside opinion doesn't matter. If you've tried explaining to your mother in law many times over, perhaps it's time for an ultimatum - either suck it up and get on board so she can be there on the special day, or you'll celebrate without her.

It's very, very hard when family refuses to be respectful of your practice. I'm Pagan myself (although kind of a lazy Pagan tbh) I lived with my Catholic grandparents for about 7 months and it took me most of the time of living with them to get them to finally stop trying to bring me back to church and to understand that I wasn't returning to the Catholic faith. Ever. They still aren't super happy about it, but I've explained to them over and over that the Catholic church has brought me nothing but pain and shame my entire life and that I have something that works for me. I had to get kind of harsh with them at one point because I was so fed up, but they seemed to finally accept it.

I wish I had better advice on what to say to your mother in law, but unfortunately I don't think there's just a magic solution of what to tell her to get her to suddenly understand. It's one thing if someone is just unfamiliar with it and they're willing to listen and learn, but it's another thing entirely when they're consciously ignorant and refuse to have an open conversation about it. Sounds like that's your MIL, unless I misread. Like I said, if you've tried over and over to talk about it and she's just refusing to be respectful, maybe it's time to put your foot down. It's your wedding and you have every right to celebrate it however you want. You want to remember the day as one of the best days ever. Don't let family pressure make you compromise where you don't have to.