Soulstorm

I will not go into great detail on this as there is another guide already being written specifically to cover Acts of Faith, this is just general knowledge to allow you to make effective use of your Faith resource.

Faith is generated by Faithful units; specifically, the Living Saint, Canoness, Confessor, and Seraphim. Missionaries can use Acts of Faith but do not add to your Faith income. You set the amount of Faith you can have at maximum by building Holy Icons on your listening posts, with the first Icon giving you a maximum of 20 Faith and each further one adding 20 to that total, to a limit of 100 possible Faith. Once you have Faithful units on the field, Faith is generated just like Requisition or Power, only much more slowly since you don't need hundreds, only a couple of dozen to start being truly effective. Let's briefly go over the Acts of Faith.

Lay Hands
Cast by: Missionary
Effect: Increases the regeneration rate of the squad by a large amount for a short period of time. This is your first Act of Faith and a very useful one the whole way through the game, a badly damaged unit can be brought back from the brink of death with this Act.

Emperor's Touch
Cast by: Missionary
Effect: Replaces all the weapons in a squad with Emperor's Touch Bolters, which do universally good damage (about 20 damage per second to virtually every unit type, compared to a regular bolter's 6-9 damage, under ideal circumstances), have a 50% moving accuracy rate (which is VERY good, most weapons only fire at 15% accuracy while moving), and have a range of 40, which is 5 more than a Heavy Bolter. Also gives the squad a cool golden aura which does nothing but looks really great.

Divine Retribution
Cast by: Missionary
Effect: Holy brimstone rains down on any unit attacking the affected squad. In effect, any enemy unit that attacks the squad with Retribution cast on it will suffer return damage in the process. Mostly useful for Repentia squads, since if cast on a Battle Sisters or Celestian squad, the enemy will just target something else until it wears off. It's a lot harder to "just target something else" when the affected unit is a battle-crazed nun with a chainsaw.

Purifying Light
Cast by: Canoness
Effect: A great flash of light that blinds the faithless. The weapon accuracy of all nearby enemy units is severely reduced for a short period of time. This affects all weapons, not just ranged weapons, so getting some Repentia into close combat with that Possessed Marine squad and then hitting them with Light is a fine way of carving through them without losing a lot of your Sisters.

Ascension
Cast by: Canoness
Effect: Summons a unit of Avenging Angels to the battlefield. The Angels are unkillable and do very good melee damage, though they are not permanent. Excellent for countering hard melee specialists like Possessed and the Seer Council, or for charging and tying up ranged troops like Fire Warriors or heavy weapon-laden Space Marine squads, since the enemy cannot kill them with firepower.

Holy Edict
Cast by: Confessor
Effect: Temporarily stuns a target enemy squad, leaving them easy prey for other nearby friendly units.

Emperor's Wrath
Cast by: Confessor
Effect: Divine ass-kicking. Inflicts immediate and magical damage on a target squad or an area, dealing pretty effective damage in the process. Good for relieving the enemy of a few hit points before taking them down quickly with Heavy Bolter fire or chewing them up with Repentia.