The Theocrat is probably the most unique of the classes in the game as far as fantasy tropes go, and with interesting mechanics to match. However the best way to play a theocrat isn't very obvious and they are one of the trickier classes for someone new to figure out, so i thought I'd write this guide to help everyone join the cause and purge some heresy. I'll go through step by step in the hero creation process discussing race choice, specializations and then into the meat of the class itself and some simple strategies for throughout the game stages. I don't play much pvp, this guide is more focused on familarizing newer players with the class than giving you pro-pvp strats.

In addition to class units that alter by race, you also wat to consider three primary unit types that the theocrat focuses on, namely cavalry, infantry, and supports. These are all units that can be given the devout property by the theocrat, which is important for getting out lots of damage.
Orcs
Orc Theocrat is probably the strongest combination for Orcs for several reasons. On one hand the Theocrats units and spells help cover the orcs primary weakness, low resisitance, (one pretty early theocrat spell gives a unit +6 resistance for two turns). The orcs strengths work well for Theocrat as well, all their units having more health makes martyrs much more useful(more on this later) and Orc crusaders are the strongest variant of crusaders in the game(they begin with the tireless ability). Most of the class units for the theocrat benefit from the increased melee strength of the orcs as well. All in all Orcs are a very strong pick for any aspiring Theocrat.

Draconians
Draconians are a rather strong race all around, but they make particularly excellent theocrats for two main reasons. The first reason is that the draconians supply a large amount of fire damage with their racial units, which gives you more options for dealing with enemies that are spirit immune(unfortunately not helpful against machines) The second reason is really just for one change the draconians bring to a class unit, the evangelist. The draconian evangelist gets a ranged, arcing, fire shot, which, considering for every other race(save humans now) it is a melee only unit, thats a pretty big perk.

Halfings
Halflings have a couple synergies with theocrat, the big one being that the many morale bonues the theocrat can offer synergize well wiht the halflings lucky mechanic, halfling evangelists also provide bard skills to add to this as well. Their martyrs also have slingshots, making them a more useful ranged unit than it is for most races. Unfortunately most of the theocrats units and combat stratigies involve buffing your units and then charging in for melee combat, which halflings can only perform well in if they get lucky(pun intended).

Humans
Humans go all in with spirit damage, with the latest patch their martyrs use a spirit blast attack instead of throwing rocks, and thier evangelists gain a ranged spirit bolt attack similar to the human priest. While they dont really add any diversity in terms of damage for the theocrat, adding range to the evangelist is always good and their knights are a storng cavalry unit.

Frostlings
The major synergies with the frostlings are one, that their crusaders and exalted have frost weapons, giving you some more damage types. Two their tier 3 unit is a support, meaning it is affected by the support uprades the theocrat gives, and the frostling royal guard can be used in place of martyrs for their supports.

Tigrans
The cat people have really only one interesting synergy with the theocrat, and that is that thier crusaders gain sun shields. Dont get me worng though, sun shields is an incredibly powerful ability, and getting a tigran city to produce crusaders would not be a bad idea.

Section still needs to be changed, I'm still experimenting with the new specializations so cannot put a verdict on them yet
Specializations for Theocrat isn't super complicated right now (although rumblings of some of the new specializatons in the next expansion may mix things up) as two of your three slots are gonna be the same pretty much everytime, for the last one some options are better than others but its mostly down to what you want.

Slots 1 and 2: Creation Adept and Mastery
These are the two that i run 99% of the time when i play theocrat, mostly because no one but a theocrat can use them nearly as effectively. The main draws of creation adept are the two tactical spells it offers. Bless is a strong and extremely Mana/CP efficient buff, at a cost of just 5 with your leader present, you can easily spam it and make many units in your army tougher, and with the morale bonus it also gives, hit harder. Then we have Holy Cure at it heals 25 hp at a cost of 10, which per HP makes it the most cost efficient adept level combat spell(fireball is a close second at a potential 22 damage for 10). The other two spells in adept are terrain spells, useful for raising happiness in your cities.
Mastery on the other hand is the section that not many outside the theocrat can use, the Strongest spell in it is Ressurect, which lets you bring back a dead unit in battle. Then there is Condemn for the Wicked, an odd duck of a spell but effective, this spell makes any unit that kills a one of your units suffer a severe armor and resistance penalty. The odd bit is that this requires you to lose units for it to work(you can cheese this by keeping some cherubs on standby to sacrifice on strong units for the debuff). Bane of the Unnatural is a powerful attack buff, mostly helpful for clearing undead dungeons, or more commonly for fighting those filthy heretical necromancers and sorcerers.

Slot three: Do what you want
Frankly this slot is down to personal choice, my personal recommendations right now would be either fire adept,(if you are running draconian this is a must for skin of oil) expander(for city founding games) or wild magic adept(dem elementals).

Here I will just run through the various Theocrat class units and give you some ideas on their role and how to use them effectively.

Cherub
The cherub is your early game summon, they have true sight, and they can fly, making them strong scouts, but low health, defense and only mediocre offensive ability makes them relativley poor in combat, they are spammable summons though so if numbers are needed they can help.
Martyr
The humble martyr, a somewhat squishy irregular, who on the surface doesnt seem special, mediocre defensive stats, a dinky physical channel ranged attack, but the real value in these units is their special ability Absorb Pain. When they use this on an ally any damage that unit would take is transferred to the martyr and reduced by 35%. Allowing your soldiers to charge heretics fearlessly and emerge unscathed. The best use I've found for this is keeping your martyrs in the back near some support units(pretty early on you will research an empire upgrade that gives all your support units healing) so the supports heal the martyrs so the damage the can absorb can be doubled, or maybe even tripled. Relatively early on you can gain a spell that makes martyrs evolve to exalted at gold rank, and they get to keep absorb pain. It's cool, but martyrs main purpose is to be expendable batteries of hp, so it's something i rarely use.
Crusader
Ahh the crusader, your frontline against heretical forces. These will be your bread and butter throughout the mid game and onward. They are a powerful, beefy infantry unit wiht strong defenses shields and a solid attack. They also have the ability Holy Champion which gives them +5 fire strength when fighting undead or devout units. This makes them effective for clearing tombs, or fighting other theocrats, and they are the bane of a necromancer's existence.
Evangelist
The evangelist, your tools for spreading your faith and bringing the heathens to the light. These are a tanky, melee only(unless you are Draconian) support unit with high spirit damage on thier attacks. Their main tricks though are their two special abilities, Touched by Faith, and Convert. Touched by faith allows you to give a resistance and morale buff to a friendly unit, particularly helpful for countering the lower resistance of orcs. The winner here though is Convert, a strength 11 spirit hit, if the enemy fail to save it, they join your army, permanently. A way to build up a growing, cheap army is to make a stack of 6 evangelists and clear resource structures and independents with them, Converting as many units as possible allowing you to fill up a few motley stacks of units to use as cannon fodd- I mean to fight bravely against the heathens and drown them in the blood of the faithful if neccasary. It is also worth noting now that evangelsits have turn undead, which was rather unhelpful before, but can be a useful way to panic undead armies.
Exalted
The exalted are a flying infantry unit with a powerful melee physical/spirit attack. Their coolest trick though is they innately have Resurgence, which means if they die, as long as you win the battle, they will revive with 50% health, allowing them to make risky plays you wouldnt use other units for and without losing them. The key when using exalted is to know when to kamikaze them, whenever you are confident you will win the battle, dont be afraid to get them killed tanking a tier 4 or diving some archers.
Shrine of Smiting
The Shrine of Smiting is your ultimate tool for burning out the tendrils of heresy, it is a floating machine, with two powerful attacks, and the damage for both of these attacks is increased for every devout unit on the field(not just one stack). It's primary attack is a ranged single target attack that does fire and spirit damage, its other attack is a single use wide AOE attack that deaks spirit and shock damage, it can easily one shot clustered stacks of lower tier units. The shrine of smiting differs from the tier 4's of other classes in that it is not a powerhouse on its own, in a 1v1 it wont be able to take the other tier 4s in combat, but with 2 or 3 stacks of devout units around it, it can deal absolutely devastating damage. Like in this screen
The one annoying bit is that being a machine, the only way for a Theocrat to heal a Shrine of Smiting is either with a dreadnought hero or sitting it in a city with a masters guild for a few turns. This However has been changed recently, and now by having devout units in the stack with it the Shrine regens using the Ancient Relic ability.

One important facet to theocrats that many new players often miss, and i don't know how i forgot to add tis section till now, is the devout trait and its purpose. There are three types of units that the theocrat can make devout, namely infantry, cavalry, and supports. The first two are made devout through the Knights of the Templar research, while the latter is done through the Order of the Sacred Support. On the surface devout doesn't seem to do anything, but thats because its power is connected to the theocrats ability to mark heretics. When a unit is marked as a heretic, devout units gain +3 damage against them, which is like gaining the damage bonus of 3 rank-ups for the unit. The theocrat has 3 ways of marking units as heretics, the earliest is the Mark of the Heretic strategic spell, at 20 mana/cp cost it is relatively cheap, and allows you to mark 1 stack of enemies for 5 turns. The second method is Denounce city,a beefier strateic spell at 60 mana/cp, but when cast on a city all units in the domain are marked as heretics, making it very effective for large siege battles. Third and most recently added, evangelists gain a mark heretic ability at silver rank, which seems late but you can usually train evangelists at bronze in a built up city so it isnt too ard to achieve. This ability is a long range, unresitible mark that makes a single unit a heretic, useful for getting extra damage on a high value target when you couldnt aford the spell cast. The damage from devout units fighting heretics is an extremely important part of using the theocrat effectively, because i mean, how can you purge heresy without shouting at them and calling them heretics first.

What you should be doing in the early game as a Theocrat is building up your city focusing on builders hall, storehouse, then a temple to make support units, and you'll want an arena at least for training crusaders later on. Then you will want to be clearing camps with your starting force. At level 2 you can get an upgrade for your hero called healing aura that greatly increases the regen of your army allowing you to clear camps very quickly. As your hero levels up you are going to want to focus on the army leader improvements for him, with leftover points being spent on first survivability, then offense. Be sure to get a few cherubs out early for scouting and grabbing pickups and just work on expanding.

A lot of people define when exactly mid game starts differently, for the sake of this guide i will define it as when you have about 3 cities, your capital has the tier 3 unit building constructed in it, you have produce crusader researched along with 50cp and a chunk of spells. Once you hit this point you are going to want to create some workhorse stacks, consisting of 2 supports, 2 martyrs, 1 tier 3 and 1 crusader/cavalry unit. The tactic for this stack is to have the martyrs absorb pain for the melee units, and then keep them near the support for healing then have the melee units do the heavy lifitng with some pokin from the supports. These stacks should be able to handle most things thrown at you for a while, although from time to time you may need to replace the martyrs, but they are cheap and expendable instruments of the cause. 2 or 3 of these stacks in addition to the swole stack you hopefully have been building with your leader should allow you to grab several cities, and possibly take out a nearby enemy.

Late game for a theocrat you have a few different options, for one you can build some Conversion stacks to go around clearing camps and gathering some cannon fodder stacks(and the occasional troll or ogre which are nice) To use as support for some regular forces to take over enemies.

Another tactic, which i like to call the Crusades, is once you can construct Shrines of Smiting, build stacks of one Shrine of Smiting with 5 things of crusaders. 3 of these stacks fighting together will give you extremley powerful attacks from the shrines, and plenty of faithful soldiers to protect them.

You should be able to fight most late game enemies pretty well, except for one, those filthy, iron worshipping, heresy ridden dreadnoughts. Their machines are immune to spirit damage, which is a considerable chunk of the damage you produce, and also adept at dealing damage to many units at a time, which counters your hordes of faithful fighters. Not much you can do you say, hard counter too stronk? Well you are kind of right, but there are ways to combat this, one if you are playing draconians, their natural fire resistance will help mitigate a the damage the dreadnought can deal with firetanks, mortars, blunderbusses and the like. But the primary way to fight those heathens machines is your global ultimate Armageddon. It makes all enemy units suffer 80% spirit weakness. That is a huge blow and will allow you to put a hurting on those stupid machines. Aside from the ultimate though, you are right, dreadnought are the counter to theocrat and will be difficult for you to fight unless you can outproduce them and just drown thier machines in the blood of the true believers.

I hope this guide, while not intended as an exhaustive step by step, has given you a much better idea of the Theocrat class and how to use them effectively. This is my first guide so leave any suggestions below. I apoligize for this just being a wall of text right now, I plan on adding some pictures to pretty it up some when i get the time.