The Glowing Guardian is a deity of justice, heroism, and an inordinate number of rules. He is one of the most powerful deities in the game, but he also has significant limitations that require conversion to use him effectively. The Glowing Guardian has been the subject of frequent updates recently, and has seen overhauls to how his piety gain and loss mechanisms work.
The Glowing Guardian is unlocked in a subdungeon to the East that will put you through several trials.
The Glowing Guardian is at his finest in two circumstances: at the very beginning of a dungeon run when you're a fresh 1st level character, and at the very end of the dungeon run when you've already spent all or your potions and need a last minute boost to pull off a win. Glowing Guardian is a significantly weaker option towards the middle of a dungeon run, where you will not be able to effectively leverage his long-term benefits but will still run into some of his late-game complications.
When joining the Glowing Guardian at early levels, you will want to maximize your long-term advantage from Humility and Absolution.
The Absolution boon allows you to convert weaker monsters into hit points. In the long run, this will only cost you a small amount of piety, but will grant you a considerable number of hit points. As a low-level character, this can easily give you a health total five to six times higher than what you'd normally have. Since you have to gain a Prayer Bead each time, the item slot efficiency for five Absolutions is twice as good as a Pendant of Health, and you gain 5% magic resistance in the process.
Because your health regeneration rate does not increase, you will need to level-up to refill your health bar. Exploration will be ineffective for healing until you've gained several levels, and with your high hit point totals you should be able to fight powerful monsters for large amounts of bonus XP.
The Humility boon is one of the most powerful in the game. Lowering your current level will reduce your health regeneration speed and the effectiveness of some glyphs that are based on character level (such as HALPMEH or BURNDAYRAZ), but will not reduce any of your other abilities. When fighting monsters that are a higher level than yourself, this will increase the amount of bonus experience that you earn. In the long run, this extra bonus XP can be extremely beneficial. To look at it another way, when you have earned enough bonus experience to pay for your initial XP setback, you will have caught up with where you were before and essentially be sitting on top of one level's worth of bonus health and base damage - this is quite easily achieved if you manage to take Humility at level 2. For longer endurance challenges like Naga City or Gaan-Telet, the extra level's worth of bonus stats on a Level 10 character is invaluable.
The Cleansing boon is more niche, but can be invaluable for fighting physical-resist bosses or in situations where you are trying to get the specialist badge. Although it's an expensive method to clear status ailments, it's less painful than using a potion.
Full-time worship - Enlightenment (or not?)
Eventually, you will need to deal with the Glowing Guardian's one unavoidable hatred: drinking potions. At a punishment of 10 piety per sip, this can be catastrophic for anyone who needs this resource in their end-game fight. The most obvious option is to convert out. Given the large amount of piety the Glowing Guardian can generate if you begin worship early, this is a fairly straightforward approach. Another option is to desecrate other altars, which will grant you indulgences to ignore piety loss. A third option is to convert your potions for piety and rely on the Guardian's protection boon, but this is usually only sensible when pursuing the Feeling Parched badge.
Keep the above paragraph in mind before you consider performing Enlightenment. If you only manage to buy the boon just before the boss fight, you will have 0 piety and will hardly earn any more until your next level-up, if ever. In this moment, you can't use the Protection boon, and you can't drink potions. Is it really worth it? What do you really want - Mana? Magic resist? Damage? Are other gods able to help with this more easily? Enlightenment scales, but just when it's starting to scale into something good, it ruins your day by denying you refills. Don't fall for the Enlightenment trap. Instead, either budget for an Enlightenment that works for you, or simply convert out.
If you can desecrate a relatively harmless altar like Binlor's, then you will have some piety and three indulgences, which can go a long way. So using Enlightenment immediately before a desecration can be a good option, but sometimes this is not available.
When else can you use Enlightenment? You have some options for managing piety. One way is to be conservative on Absolutions, doing 2-3 of them instead of 4-5, taking Enlightenment a little earlier, and then continuing to level up for more piety. This takes a lot of planning, and you will likely end up having to convert potions you can't afford to drink.
The last option is to just farm that much piety - but only the longest dungeons make this feasible. The idea is to gain Enlightenment, gain 50 or more piety, spend the excess on Protections, and convert out so you can drink potions at your leisure. This won't really be feasible without either a very long dungeon or the presence of Pactmaker for the Consensus boon (immediately after you use the protections.) Even then, your new god won't do much for you unless it's the kind of god who awards piety for killing popcorn or using blood pools in some way, but the important thing is that you can use potions.
The perfect storm, then, is a long dungeon with Binlor and Pactmaker for pure Enlightenment shenanigans. The most likely way to pull this off is to prepare Binlor, since preparing Glowing Guardian himself incurs a penalty that defeats the purpose of full-time worship. In otherwords, extreme Enlightenment is a cool random opportunity you may stumble upon, but not a feasible strategy in general.
The TL;DR on Enlightenment is that it isn't always a strong option, and taking it at the wrong time can easily ruin your entire run. Converting out at 50-100 piety is often for the best.
The exception to the rule is Paladin. A Paladin can spam Absolutions, get Enlightenment on a discount, and then drink potions at her leisure, incurring no punishment for it. Aggressive use of Absolution and some Protections, saving just enough piety for an Enlightenment near the end, is actually viable in this case.
One of the best ways to use the Glowing Guardian is to convert into his religion after having expended all your potions in order to spend piety on Protection. This can be an effective last-minute boost, especially if you have plenty of items in your inventory that you can convert for more piety.
The Glowing Guardian works well with several deities. The most straightforward ways to convert into his religion are from The Earthmother or Binlor Ironshield. Both of these gods can generate large amounts of piety for 1st and 2nd level characters, allowing someone to arrive in the Guardian's religion with large amounts of piety (and perhaps a few boons already purchased) but still a low-level character. When converting out to other gods, there are two particularly powerful choices: Tikki Tooki and (perhaps surprisingly) Dracul.
Both Tikki and Dracul are powerful end-game gods that can empower a character who has been leverage the Glowing Guardian's long-term advantages. Both suffer somewhat from the use of absolution, since it removes weaker monsters that can be killed (and in the case of Dracul, harvested for blood) for piety. However, both of these gods can provide benefits that are extremely well-suited to a typical Glowing Guardian follower.
The Glowing Guardian does not work well with Taurog, Mystera Annur or Jehora Jeheyu. Due to inventory constraints, it is very difficult to use the boons of both Taurog and the Glowing Guardian. Mystera offers no particular synergy with the Guardian, but they will at very least not interfere with each other. Jehora Jeheyu is generally not a god that favours conversion between religions to begin with, and given the windows of opportunity for optimal conversion are quite narrow with the Guardian it's seldom an option to jump between these two gods.