Frank Mentzer on religion in BECMI, from a facebook post

Frank Mentzer J Conrad, I think a more detailed answer is appropriate.

To address your point -- Belief Systems, aka Religions.
I very specifically avoided all mention of any major real-world faiths in the entire BECMI line.
I avoided derivatives of their beliefs, too. (Compare to extensive Christian influence on Gary's works; he was a Jehovah's Witness, and I'm agnostic.)
I also avoided lesser-known Beliefs as much as I could. Frex, some people still believe in Astrology or Phrenology, so I didn't go there.
I toyed with a religious setup that was 100% fictional, but it turned out indistinguishable from Parody, so I stopped.
You want details of the religion in your game? Fine, please make your own. But don't mock others.

Underlying: Think about it. In this fantasy world, high-level clerics know what comes after death. Some of them chat with gods/immortals on a regular basis. They also control health care, and probably education. So why don't they take over? The Only Possible Answer: the gods/immortals rule that off-limits; if you covet mortal power, leave the faith and start a Dominion (and forget about cleric spells). It's a matter of multiversal balance, preventing excessive power from congealing in one place (clerics) and ruining the balance of Everything... thus upsetting the Prime Plane in general, and thus decreasing the likelihood of getting more Immortals thru ascension.
So for the gods/immortals, it's just a matter of self-interest. It's about getting more Immortals.:)

The sprint button in DOOM 3

I have discovered the sprint button in DOOM 3, and my experience has been enhanced 1000 fold. The trick is to realize that every time you engage in combat, you must mash the sprint button. Suddenly it becomes possible to outmaneuver the enemies and actually dodge bullets. It's possible to shoot out enemy projectiles in DOOM 3 but it was a feat I had only accomplished with the plasma rifle before. With the increased speed from sprint, now I can shoot out fireballs with just the pistol.  I had always found DOOM 3 to be atmospheric as fuck, but with the faster movement speed now I greatly enjoy the combat as well. The only downside is that the speed is on a limited duration,  which really shouldn't be a thing on a DOOM game, but no game is perfect.

And I'm highly excited for DOOM: Eternal

Challenging the Players

I went into AD&D with the notion that if the players accurately described what they were doing, I would allow them success or failure on their task depending on their description. I thought this would be a refreshing change from modern D&D, where everything is resolved with a skill check and a die roll.

Unfortunately, this did not work in practice. My players simply described what they were doing and blazed through most challenges with nary an effort. How would you adjudicate climbing a tree in AD&D, without the Thief's climb skill? Surely you don't need to specialize as a thief to climb a tree. 5e's athletics or acrobatics skill would be an easy fit, but if I use AD&D's climbing speed rules and give my player an instant success on the climb attempt, the loss of a few segments of time is hardly worth tracking and what should have been a mobility challenge just became a few seconds of wasted time IRL.

 This is especially true for the most common dungeon challenges - locks and traps. By letting players describe 'alternate' methods of cracking them open and disabling them, I merely taxed them by only 1 turn of action, and so they moved past the challenge virtually unhindered. The occasional wandering monster did little to impede them. On the other hand, D&D 5e would approach these challenges with a die roll that has to meet a target Difficulty Class number. Failing die rolls is often frustrating, but the hidden upside is that it causes a natural deterrent to player behavior. If they fail a die roll multiple times, in the AD&D system, that means that they're losing multiple turns. Which means that other players are doing other things, and there are more frequent wandering monster encounters.

Sadly, it seems like I'll have to implement more dice rolls in my game. A 1-in-6 chance or % out of 100 seem like the best fit for AD&D. The Difficulty Class system is a better fit for BECMI, since it uses attribute stat bonuses anyway.