I suppose the tradtitional set up of a service isn't very welcoming in a lot of churches.
You file in, find a seat, there's a bit of quiet chatter then a hush. Everyone faces in one direction as if in school again.
In some services you might be expected to be able to read either some prayers or songs. If you struggle with reading then you'll stand out unless you "know" the order of things. That presupposes a background of attending church.
I wasn't allowed to go to church until I was 18. I'd only ever set foot through the doors of a church for school or Brownie services and it was completely alien to me.
I didn't know which bits to join in, when to stand etc. Obviously in a reformed church there's a lot less to "learn" or recite than in an Anglican or Catholic church, but there were still bits like the Lord's Prayer and a couple of sung Amens here and there.
So my unease wasn't because of my class or level of education. It was about the assumptions of club membership.
I've been to other churches since (my centre of Glasgow one is ace at this) where things are explained before they happen. Either in the bit of paper given out at the door, or by someone up the front.
And everything is always prefaced with something like "We will all..." or "If you are able to..." etc.
On the other hand I've been to more traditional churches having been a Christian for years where I've felt alike a fish out of water (no pun intended!). I've felt like a gatecrasher or like I'm attending aservice for a different faith altogether!
I supsect IME,the biggest barrier to anyone walking into a church service is having never been to one before and not knowing what's going to happen, rather than a class thing.