If you start from the assumption that Christianity is true, and some people know this, and others don't – you have to ask why the people who don't know it, don't know it. And this is where Catholic theology distinguishes between "vincible" and "invincible" ignorance - "vincible" means the ignorance is your own fault, "invincible" means your ignorance is through no fault of your own.

How to distinguish the two? Ultimately, it is up to God to decide – nobody else knows for sure what's going on in your head. At best, theologians would give some examples of hypothetical situations which could be said to be one or the other – but the real world is often much messier than any such hypothetical can capture.

Which is part of why, the traditional Catholic teaching, is that (with rare exceptions) you can't actually know where people are going to end up. The idea is that if you make it to heaven, you might be surprised to find a lot of people there you weren't expecting, and also maybe some people you were sure would be there aren't.