To be clear, is he an expert on the afterlife, or does he just know something about it because he's been there (so, he's an expert, compared to everyone else, who... presumably... haven't been there).
Put another way: are you an "expert" on your closest city, just because you've been there often? (usually, tourist guides will know much more about the "sights" than residents).
I'll assume you mean "expert".
As others have said, make other characters react to him as if he was an expert. Reactions are very often used in this way e.g. make the hero attractive by having attractive girls gaze admiringly; high status, by people deferring; low status by ignoring, treating badly. Etc.
There are other characteristics of an "expert": they will use ordinary terms with technical meanings, that confuse laypersons. They won't define their terms, but just use them to answer questions correctly, (seemingly...) without realizing that their explanations don't make sense in terms of ordinary meanings. This belittles the layperson, and the expert's status. Very importantly, the expert really is making a valid point; just... not communicating it. Listening, you do get a sense of expertise, and that some words are not being used ordinarily.... but you don't know which ones, or what the true meanings are, how they relate, or their significance.
For example, there could be different specific technical meanings for die, pass away, decease, etc.
You can also make him right about a prediction, or masterful in a skill. But this is more a real proving of expertise, that takes some time to setup, and (almost) nevers happens with experts in the real world.
Another approach is how you yourself evaluate experts. How do you know your medical doctor is an expert? ("Dr", role, dress, attitude) dentist, lawyer? Builder, programmer, hair dresser? Maybe you google them, ask other people (esp if they are something of an expert in that or a related field).
When you an "expert" in a film, on TV in a novel, what made you feel they are an expert?