That's pretty much what happened to me. Well, minus the protestant part, I grew up Catholic of course, the majority of my country as well as Europe is Catholic.
But yeah, over the course of growing up I just progressively stopped believing more and more in Christianity, until I only believed / semi-believed that there could exist some entity or force and that we have a soul.
But at some point, and this was a big eye-opening moment for me, I thought "but why do I believe in that?", I never had question that and the answer was clearly because other people told me so. And... that's not a great argument is it?
At this point I was already interested in Buddhism and it has remained so to this day.
I could not have worded it better myself!