Since no one returns from death, we can never be certain about what pa

First off, this is a type of LSAT logical reasoning that doesn't have a direct counterpart on GMAT critical reasoning, so don't expect to see a question just like this on the GMAT. Inference, assumption, strengthen, weaken - those questions all exist pretty much identically on both the LSAT and GMAT, but the LSAT has a few question types (those involving formal logic and the "contrapositive," these that ask for the main point instead of to draw a conclusion or strengthen/weaken a conclusion) that extend past the scope of GMAT CR.

That said, there are two pretty big context clues here that can help you learn lessons applicable to GMAT CR and RC. In finding the main point of the argument, I'd look at:

1) The word "however" - a common line of rhetoric involves an author bringing up a generally-accepted or at least logically-coherent point and then using "however" to transition to a more narrow/recent/proposed main point (e.g. "in general, people believe X. However, recent research suggests Y..."). The word "however" is a good place to look both in CR and in RC for a shift from conventional wisdom or introductory context to what the author is really trying to get at.

2) The sentence "Why should we think so?" An important method for finding an author's conclusion in a CR context is what we call at Veritas Prep the "Why Test" - in order to be a conclusion, a statement needs to have something else in the paragraph explaining why that's true. If there's no explanation of why, it's just given as a premise or as context. Here the author even uses the word "why" to show you that he's using the next sentence to explain the previous one as a claim he's making. That signals that he's building a case for "it is quite possible that death presents itself as a glorious experience."

So look out for these rhetorical devices: the word "however" is commonly used as a transition from background information (or a conflicting viewpoint) to the author's main point, and words that signal evidence (because, since, or a sentence saying "why should we think that?") can signal that the author is about to give reasons why his main point should be believed.