I was lucky that, despite being somewhat religious, my parents were fine with me being an atheist. We would even debate the merits of religion and they did not have any issues with my questioning of their beliefs. Both were Lutheran and they had raised me in that tradition. I went to Sunday school, attended the Lutheran Catechism and reached the point of Confirmation. And that was right about the time I realized that the whole thing seemed to be based on a bunch of old stories with no more evidence than elves or faeries. And that was always the crux of my issue with their religion, and one they could never argue past.
When it came to my kids, they have been raised with my complete lack of belief and my wife being agnostic. We spend our Sunday mornings sleeping in and not going to any sort of church/temple/forest altar. Though, that last might happen, if it’s ruins at the end of a nice hike. My parents never expressed any disapproval and the lack of religion was never an issue. Technically, my mother is still kicking about and could suddenly go off the deep end, though I strongly doubt that’s in the cards.
At the same time, my wife and I had discussed religion before we had kids and what we might do in the event it became an issue. The simple answer was, “fuck 'em”. I love my parents, but my kids come first. If my parents had decided to get stupid over us not indoctrinating our kids in their fairy tales, then I would have just removed them from my life a few years before death did it anyway. Sure, it would have meant the kids never knowing their grandparents. But, there are lots of assholes in this world, I don’t see the need to personally inflict them all upon my children.
The best thing you can do is talk to your partner and have a plan. I would say that, if you expect it to be a point of contention with your parents, you might want to talk with them about your views on religion before it gets to that point. It doesn’t need to be anything confrontational, just be up front and say, “I don’t believe what you do”. You don’t need to go on a Dawkins style, “your religion sucks and you are morons for believing it.” Just make it clear that you don’t believe. It’s still entirely possible to have a warm, loving relationship with folks who don’t believe as you do. It just requires that each side treats the other with basic human decency and respect.
Expect a lot of boredom.
While my experience is from different states, I’m pretty sure that part is universal. Bring a book. Listen to instructions and just go with the flow. But mostly, expect to be bored.