I’m sure you do, and he may well be a great guy and maybe this fabricating things for his family is just the way he has learnt to cope with them.
BUT you can’t always predict the kinds of deep rooted feelings and values which will come to the fore when you have children, we are all marked by our upbringings. And that can be hard if you’ve rejected the values you grew up with, but it’s even harder if you can’t actually admit that’s the case.
You also have no control over his family - what if they decide to visit? what happens when you take your (future) children to visit them? What happens at every religious event, celebration, milestone? What kind of message does it send to your children? I take the PPs point about children identifying as part of a specific culture but not necessarily being actively involved in it, but as she also says - that can take generations. Your fiancé isn’t there yet really. How can you invite your children to make a choice about their religion, or bring them up without religion, EXCEPT when they have to lie to family members about it? What happens when they want to have relationships with their cousins/GPs/aunts or uncles? Will that be feasible if they have to keep pretending they live a way they don’t?
What happens when your fiancé’s mum insists that some particular thing should happen or is dealt with a certain way - he can’t be honest about his life, will he be able to say no to her about other things? Or will it be a constant ‘well we can’t upset them so let’s just go along with jt’.
At the very least you and he need to talk 100% openly about every eventuality you can think of and figure out if you’re on the same page about how you respond… not because those responses are set in stone, but to see whether you are going to be able tolerate each others’ attitudes to this long term. This really isn’t a case of ‘well we love each other and we will work it all out eventually’. You need a more tangible plan than that.
Marriage and children is a tough enough gig when you are on the same page and pulling together and you have each others’ backs. How are you going to make it work if it starts out like this, and you’re with a guy who can’t be his own person on some pretty major issues that also impact you?
It doesn’t necessarily mean he’s not a good guy, but it definitely means he isn’t mature enough to handle forging his own path through life…