So here is a game I was playing where I happened across an early Glowing Guardian altar. GG is now one of my favorite deities, but as a beginner I found him to be totally confounding. There are two main approaches to using GG:
The Lifestyler: This is probably the way that GG was intended to be used. You join the religion and stick with it until the bitter end. You will take Humility at 2nd level, Enlightenment whenever you amass 100 piety (probably around level 6-8), and Absolution right before entering level 7 or 8. Absolution offsets the loss of popcorn harvesting, and Cleansing + Protection offer a workable (if weak) substitute for potions.
The Sunday Worshiper: This is my preferred approach. Your goal is to join, get the low-lying fruit, and then quietly sneak out the back door. Take Humility right away, Absolution at 2nd (optional), and then convert to someone else as soon as convenient. It doesn't matter who your new patron god is; you are converting
out more than you are converting
in.
If you are lucky enough to find an Agnostic's collar, you have a third option
The Apostate: Join GG and take Humility as usual. Use Absolution at 2nd (optional). Pick a god you don't like and desecrate his/her altar (do this twice if you are using the 4 altar prep). You should be able to get Enlightenment very early now. Once GG starts cramping style, convert to the god that you didn't desecrate, and then desecrate GG's altar for good measure.
This is a very powerful option that nets you the two best boons in the game (Humility and Enlightenment), without saddling you with Glowing Guardian's crippling restrictions. You can sometimes pull this off if you find an early Binlor instead of the collar, and happen to stumble upon GG's altar while still exploring. I'll talk about how that works in a different post.
So anyways, I was playing as an elven berserker, trying to complete the Unlikely Heroes quest when I found an early GG altar. Because I didn't start out intending this to be a demo game, I am carrying a lot of advanced preparations. Don't worry too much about them, I hardly used them at all.

A perfect set up. Glowing Guardian found at level 1, and several easy higher level targets just asking to get their faces smashed. My first move is to kill the Warlock-2, which both levels me up and gives me a nice chunk of piety.

Things get a bit complicated here. It would be a huge waste to kill this dragon before taking Humility, but I don't want to take the boon just quite yet. Instead, I bring the dragon down to one hit...

... and then fireball the wraith.

Now I take Humility.

And kill the dragon for a pile of xp and piety.

And then I finish off the wraith for another pile.
While Humility doesn't lower your stats, it does lower your actual level, and fireball damage depends on your level alone. By fireballing the wraith before using Humility, I did twice as much damage.

Once you have Humility, you should never fight anything less than two levels above you. Right now, I'm feeling torn. This serpent is a decent target, but I am confident that I could handle a level 6 monster if I just knew where one was. In the end, I decide to settle for the serpent. The deciding factor is that the serpent will poison me (which grants me piety), but will also level me up, which cures the poison and basically gives me an extra 2 piety for free.
I didn't show it, but I used my mana to fireball that go0 blob-7 right before killing the serpent. I really, really doubt that I'll be able to take the blob, pre-damage or otherwise, but it's better than wasting the mana.

It doesn't end up mattering, because Patches ends up teleporting me into the black space, instantly healing the blob. I can't complain, because he also landed me right next to a much nicer target, a wraith-8. Between the 4 level difference, the manaburn (which will instantly heal), and the special undead bounty, I'll get lots of piety for this. The 20 bonus xp is nice too!

This was an interesting puzzle. Patches mana burned me on my last level up. As you can see, I can
almost take that Warlock-9. If I could just cast
one fireball I'd have him, but I don't dare anger Glowing Guardian by drinking a mana potion. For the almost the first time in my life, I wish I had prepped a burn salve!
BTW, see those grey sparkles? That means that Binlor is behind the Warlock. Binlor is a very good deity to 'escape' to, but for this game, I decide to stick with Glowing Guardian instead.

I try regen-fighting him, but that effort fails. However, while dancing back and forth, I find both a HALPMEH and an ENDISWAL glyph. I convert all my glyphs except for HALPMEH, and then buy and convert as much shop junk as I can.

From there, I can convert 2 mana potions to reach 100 piety.

Which means that I can get Enlightenment.

Which means that I can now cream that Warlock, mana or no.

Patches rewards my resourcefulness by mana burning me
again. Once again, I find myself just a few spell castings away from being able to handle that other level 9 monster.

At this point, I could just ask GG for a Cleansing boon, but I have a better idea. I know that there is an apothecary somewhere, in this dungeon, so I go looking for it.

Crying tears of shame,
I actually buy a burn salve.

And then I rock that goblin's face. Patches responds by poisoning me, which is just free piety for me, since I have HALPMEH. I also see that the boss is Aequitas, a hilariously mismatched opponent for my Enlightened Berserker. I could end the game now, but let's pretend that I need every edge that I can get and see if we can continue milking GG for more boons.

I don't want to fight the bandits. The extra xp isn't worth losing all of my resistances. I can't harvest the popcorn because GG will go nuts. It's time to use Absolution.

Poof! Notice that Absolution raises your health cap, but not your
actual health. This is why you should always use Absolution
before leveling up, not after, even if it means leaving a few monsters behind.

The boss is totally screwed. I don't need to use any potions or boons to end the game.