Re: Catholics, I think it varies. My family is full of avid readers, but we just don’t mark in books. Nothing against those who do. Marginalia, glosses, and highlighting are honorable practices.But we try not even to crease the spine or pages, so you can see that my family wasn’t going for marking up books with notes! (And yes, sometimes it’s embarrassing to explain to an author that is signing a book that in fact it’s not a new copy, but in fact has been reread a great many times.)
I think the only time I’ve ever marked up a book was back in the 90’s, when the publishers were shifting between printing systems and huge numbers of typos and spellcheck errors were appearing routinely in books. I would very carefully make corrections, trying to go with the font to make them invisible.
More recently, I’ve had some big projects that actually necessitated marking a book with a highlighter, and it was pretty traumatic at first. Luckily the paper was pretty yellowish to start with, so I could tell myself that the yellow highlighter wasn’t that bad. It’s actually faded into the pages now, so it’s almost like new. Except not, alas.
So yes, when I first found out that people marked up their Bible, which is to say, a book even more precious than other books, it sounded really weird. Also, it sounded logistically impossible, because most of our family’s Bibles didn’t really have margins or spaces between the lines.
So yeah, mostly I make mental notes. Haven’t usually needed any other kind. But when I do need to, I now have electronic copies of my sources, which are a lot easier to annotate without destruction.
However, it should be noted that the original, normal way to write out or print a Bible was to put in all sorts of interlinear and marginal glosses on meanings, patristic quotes, etc. The way Protestants mark up their Bibles would appear to be a survival of this medieval Catholic habit.
So… if you want to mark up a Bible, mark it up. But if a Catholic’s Bible looks unread, they might just come from a neatnik family where books were always held precious. There’s no way to really tell.