Holy is an unusual path of Magic, even more different from the Elemental and Sorcerous paths than Blood. "Holy Mages" are called Priests, and holy magic sites are known as Sacred Sites.
Holy magic is powered by faith, rather than by the turning of the Spheres or magic rocks. It's a bit like Blood in this regard, powered by those involved in it; Pretender Gods cannot use it, for example, since they don't pray to anyone. Holy magic also doesn't provide Research points for understanding spells of the other nine paths. Despite all of these shortcomings, Holy is a critical path, especially outside of battle.
Priestly spells are classified in the school of Divine Magic; which, like in Dungeons & Dragons, is not discovered but granted. The capabilities of a Priest are as follows:
Banishment and Smite are colored by the sort of godly powers the Priest's Pretender God has. If the Pretender God has 4 or more levels in a magic path, this is reflected in these divine spells. If the Pretender God has multiple paths above that threshold of power, the one with the highest skill level is preeminent; if there's a tie, the leftmost one is used.
There aren't any Rituals in the school of Divine Magic, and there are very few national rituals that require Holiness. However, Priests have many functions in the strategic map that take an entire turn; you could view these as Rituals, or perhaps more similarly to other functions.
Priests can Preach the Word of God to spread their Pretender's Dominion in their province. The chance of success is 30% times the Priest's level, -5% for every point of hostile Dominion in the province (down to a minimum of 5%), with chances over 100% being treated as one guaranteed chance (per 100%) and another sub-100% chance. If a Temple is there, the chance is increased by 15%. The ceiling for how high they can push local Dominion is twice their Priest level, plus 1 if there's a Temple.
Priests can also expedite victory in-game by Claiming Thrones of Ascension. In games where the goal isn't specifically to destroy your opposition, these Sacred Sites scattered across the map will be the victory condition, but merely having them within your borders is not good enough. Claiming a Throne is a turn-long process that can be done by either the Pretender God's physical body (sitting on the thing), or a 3 or holier representative. If you're really lazy, a random event in 6 or higher (two in 8, three in 10) may feature the local populace or the province itself doing the job for you, but hoping for a province-specific random event isn't a great idea.
Priests of certain nations may Perform Blood Sacrifice, for a quick shot in the arm to their Dominion. The amount of Blood Slaves they're allowed to sacrifice in a single turn is capped by their Priest level, and every Blood Slave sacrificed does the work of one Temple that turn. For Early and Late Mictlan, Temples don't work, so Blood Sacrifice is a must! Blood Sacrifice can only happen at a Temple, though.
Last but not least, only the prayers of Priests work for Recalling God. A dead Pretender God may be summoned back to this world in a "research project" style, with Priests devoting turns to put points roughly equal to their Priest level towards the Recall. 50 points are required, so having just a 3 priest and a 2 priest on the job would make it take around 10 turns.
There isn't "Indirect Magic" from the Holy path in the traditional sense. However, those with at least 1 are always Sacred; this means they can build Temples, be Blessed, and charge half as much upkeep. (Sacred units can only be hired in provinces with Temples, by the way.)
Boosting a unit's Priest level (their Holiness) is no trivial matter, for the path cannot be Empowered and has tough-to-acquire Boosters. Every nation can name one Prophet, however; this will elevate them to 3, or provide +1 if they're already at that "master" level. If the Prophet dies, the nation must wait six turns before they can name another.
Magic items that boost Priest level (by 1 each) are the Ring of the False Prophet, the Sword of Justice, the Sword of Injustice, and the Immaculate Shield. Ragha also gets a single national Holy booster, but it can only be applied to one of two units.