You have to address this. It's unfair to everyone. Are you going to raise your kids to lie to grandparents, or are you confident enough to raise them in your beliefs of inclusivity.
But if you want a relationship with the grandparents - it's going to have to be open and honest - with them having the ability to teach them the joys of religious beliefs and not control them through religious fear or coercion.
The grandparents have a lot to offer.
You have a lot to offer.
If you want a relationship with the grandparents you need to be open and honest - and very much so, set your boundaries - or keep your boundaries flexible - but they are confined to the grandparents only stating "We believe that this is what ..."
But I also suspect your husband has made a few compromises to be with you.
Religion is not the cross he wants to die on. But his parents are important to him.
So why not make an effort to understand his parents' religion, and what teachings are important, and involve your future children as a part of their education. (They should always have insight to think their own minds and, if needed, be able to articulate defending their grandparents, even if they don't agree. That's called an education.)
I have no affinity to any religion. But I've been involved in celebratory events in many religions. I've brought in speakers to speak about many religions.
Your child possibly has a unique opportunity to understand someone else's religious beliefs. People who will value and treasure her.
But listen to what they'd ideally like and events and stages.
I went through the Catholic First Holy Communion ceremony. It didn't make me grow up to be religious, unlike most of my very loved family. But I did get a glimpse into the joys and strengths of religion.
It's an insight.
Religion is a double-edged sword. People believe what they want to believe.
And people will always seek to control other people.