But there's another category of things that can be known. There are many everyday things we interact with and
know. Gravity. (That we can "overcome" gravity, as in a plane or rocket, doesn't count. We're talking everyday stuff, here.

) Eating will sustain us; not eating for long enough will kill us. Same for breathing. Love.
We can even know some things that aren't 100% reliable, in a sense. We know a light switch will work, even though it won't, once in a while -- circuit breaker, bulb burned out, etc. Our car will almost always get us where we want to go. We
know that these things work under normal circumstances.
But the category of things we can know or not that NfS is talking about is very different. The afterlife is not an everyday thing we deal with in our normal lives. When someone says the know about the afterlife, and give us details of what will happen and what it will be like (even if it's "so wonderful we can't even imagine" or some such), they only "know" it in the sense that they have decided to believe it's true. The
only reason they do that is because they can read about it in a book, and the book can do no better than to make assertions and require that the reader accept them.
OK, I guess there's another reason: they
feel it's true. Maybe they've had dreams, or they heard voices. They will likely not accept the obvious fact that these things cannot be known to be genuine. To do so would cast doubt on what they want to believe.
Which is fine. I guess. Until these beliefs lead to heartache, fear, and despair. Which they often do. How could they not? Not only can no one actually know (except in the sense above, that they have
decided to know) what happens after death in general, they really can't know what specific afterlife they will experience -- did they meet the requirements for a good afterlife? And even, in a way, worse: they can't possibly know whether their loved ones will have a good afterlife. It is not possible, and any honest believer will agree. You
have to wonder if your child, say, will spend eternity in hell.
Well, I don't. But "believers" do.