system
Given the following post, respond as if you're commenting on Reddit:
human
[Doom] Is there a heaven? How correct is religion? For that matter, are there aliens? Do they have their own hell?

We know that hell exists nowadays, because we've been there

But how did ancient peoples find out about it?

Was there an ancient equivalent to DoomGuy?

How much of cosmology did religion get right?

What's the situation on aliens?
gpt
It's likely that in the past demons were able to cross over individually, but the lack of portal technology from the UAC or other corporations left them unable to pull off a sustained invasion.

From the Doom 4 continuity, Doomguy is also the ancient Doom Slayer, a powerful being from the realm of Argent D'Nur that resisted invasions from Hell until it was betrayed and absorbed into the realm proper. It mentions how Doom Slayer was blessed by the Seraphs (a type of angel), implying that Heaven exists, yet it also mentions that he was too impure to return to it (likely because of all his killings and the fact that he enjoyed them, no matter how justified they were)