: Life is finite and precious, thus true immortality is what you leave behind.

The one-sentence summaries sound like you're talking about, not really about the story exactly, but the message, the point. To paraphrase Ursula K. Le Guin, it's a story, not a fortune cookie. I think ZackDark is on the right track about it being about looking for something rather than being or having or finding something.

To go with your request though, I'm thinking it would be a message that weighs to one side but shows complexity, or a question to the audience, "You should have faith in people-- but be careful."
There's a lot of people trusting or mistrusting other people, and for the most part either their initial trust is well-placed or they turn around and find they should trust someone they didn't trust before, but in some notable cases that doesn't happen.

Osiris's initial and later trust in the Chief and mistrust of Cortana is well-placed, and of course their mistrust in-between isn't. Halsey's trust that the UNSC would listen to her, pick her up, do something about the Guardians and Cortana, and let her help is also well-placed. The mistrust of UNSC/ONI command towards the Chief and Blue Team is also sort of misplaced given that they want them forcibly brought in not long after failing to comply with orders, and Blue Team had good intentions the whole time and tried to carry them out. The initial mistrust of Governor Sloan towards Osiris is turned around over time. The same thing happens with the Arbiter and the Swords of Sanghelios.

Osiris's trust in Governor Sloan is dealt a blow when they find out he knew about the Guardian in advance and he'd talked to Cortana and not done anything except try to be prepared, and again when he joins up with her (I think there was also talk of him abandoning his people relatively early in the Guardian incident?). The Chief's trust in Cortana is misplaced, although he seems to figure that out pretty fast. I'd say the weighty exception (although Chief's side probably should've been weightier) is that Cortana's trust in the Chief ends up being misplaced because he doesn't go along with her and wants to stop her.

There are more instances of trust being the wise course, and they tend to include more big story beats, but the importance of it possibly not being the wise course is shown too.

(You know, after thinking over this, I feel a little better towards the story... just a little.)