The main things about Theocrat are the Devout mechanic (it's a negative trait for anybody who isn't a Theocrat.for a Theo it's an amazing buff), very good access to healing units and the ability to convert enemy units.

Oh and high level Theocrat hero rocks, not only does it has many army support upgrades, but it also gets Convert, healing, and lateron gives the whole stack Resurrence! Or you could play with the race that has your favourite support unit, since Theocrat as 2 good researches for support units (healing and +1 rank).

There are some details I missed, but all in all I found playing them as Orcs or as Halflings to be very good.

Or a Theorcrat hero that gives Resurrence to the whole stack. Just be sure to have either a repair unit or a city with an upgrade that repairs machines. The tier 4 unit becomes stronger when more devout units are on the battle field and is really powerfull. Tier 3 angel is always handy as a meat shield, cause as long as you win, they will never die (Resurrence or whatever it is called). That tier 3 class priest is also very good when playing as hafling (get minor bard skills) or when playing as draconian (get fire bolts). Their Tier 2 Crusaders (infantry) are very good, especially when playing as orcs since than they get tireless. Tier 1 Martyrs become simple but versalite archers when playing as Halfling. They are numerous spells that either mark enemy as heretic (devout deals more damage to them) or buff devout units (e.g. Their support gets healing and their infantry and calvary becomes devout after some research.

Age of wonders 3 elven theocrat