The less common ground you have to work on, the more opportunities for conflict to arise. And then you have to decide, what’s more important-- my belief in x, or my relationship with y.
Me, I married someone who was baptized Methodist, and was non-practicing. When we dated, he would go to Mass with me, and we got married in the Church. I didn’t think it would be a big deal, but it ended up more complicated than I expected. For example— NFP. I’m like, “Hey, the science.” And he’s like, “That’s baloney.” And I’m like, “Uh, science. Science is good, right?” and he’s like, “Eeep! I’m scared we’re going to have kids I don’t want.” And I’m like, “Uh, I thought we were going to work on kids in the future, right? Like, settle down and have a family?” and he’s like, “You know, I kind of like things the way they are. But in the meantime— eeep! I’m scared!”
Or, for example-- saying prayers with kids. Sometimes, even though I knew intellectually he was raised Methodist, I wonder if he’s really agnostic. Or atheist. Or something. Because faith plays no visible role in his life. And so sometimes, doing ordinary things like saying bedtime prayers with the kids— you kind of wonder, in the back of his head, “Does he think I’m being silly? Or dumb? Or superstitious?” And I shouldn’t be thinking about what DH thinks about, say, me doing a rosary with the kids. I should be paying attention to other things. But it’s still there in your head. Because you don’t have that common ground, but you do have that vulnerability that comes from wanting someone you love to approve of you/not sneer or look down on things that are important to you.