Faith has taken a different path from the Witch Hunters codex and even the versions before that. More so than just the mechanical approach of faith points and Acts of Faith. The rules have changed and along with that are the ways we use our Faith.
Take a typical Witch Hunters army. One of the principal ideas when putting together a list was to maximize the amount of Faith an army had at its disposal. One motto of mine was that if a unit was not Faithful, it did not get on the list. The sole exception to this were my Exorcists. Faith was everywhere. Faith was key to a flexible army. Faith was the hammer. Faith was an ace in the hole.
Lately I have had to think about what I was doing with my Faith Points. Faith has changed, but did I change with it? Unfortunately I had changed but not in a good way. Often my Faith went unused, which in and of itself is no big deal since there are times when you have more faith than you can spend. With Saint Celestine and Jocobus being Faithful but having no Act of Faith, there is no need to save points for them. Add in Battle Conclaves, Exorcists and the Penitent Engines who can not use Faith then top it off a few troop choices spending time in the rear hiding in their box for several turns and no one has an Act that would be of use. But there have been times (way to often) when I simply do not even roll for Faith Points, much less forget to spend them. I did not set out to design a Faithless army, but it feels like that is the direction I am headed. This made me look at how things are evolving organically, whether or not that is a good thing and how I can best support its proper use.
At d6 Faith Points per turn (my turn only) I should average 3.5 points of faith I could use. Right off I can see that if I wanted anyone that could use Faith to take and Act of Faith each turn in order to maximize the army's ability, I would really need to limit the number of units with an Act of Faith to no more than 4. Even for a 2500 point battle I could never get more Faith Points than I do with a 500 point battle. 4 Faithful units. For me that is 2 Dominion squads and 2 Battle Sister squads. These two squads have an interesting relationship in that the Dominions will be the first to fire, scouting in or outflanking to get in close quickly. Meanwhile, the troop squad will be held back in order to avoid loosing my units needed to capture objectives and also to keep from handing my opponent free kill points early on in the battle. They make good mop up troops with their rapid fire and flamers but they can not take return fire well and collapse under assault from just about anything.
Now that I have a number and an argument about why that number exists, is it the best choice to build for this untested sweet-spot? Acts of Faith will only work on average 50% of the time. 33.3% base, 50% with a Superior or after taking a wound, 66.7 with both. It is possible to bump this up to 83.3% by adding one of the named characters to a weakened unit that has a Sister Superior. I can see Saint Celestine joining a Seraphim squad for an assault ... maybe. Most likely not, but that is a different tactical decision than what I am think (out loud) about here. So, Acts of Faith are 50% reliable.
Also is should be stated that not all Acts of Faith are equal. While one can let you re-roll and 1's, another (twin-linked) basically lets you re-roll and 1's and 2's. Without breaking down and giving away the rules, this is what I have as Faith priority:
Repentia about to assault
Seraphim shooting
Dominion Shooting
Sororitas Command if needing to move
Retribution Squad shooting ... maybe.
Battle Sisters shooting
Celestian assaulting
Canoness assaulting with another unit
This list is hard to prioritize and it will be prone to change depending the situation for any given turn. I can not see taking a Celestian squad at this time. No matter the role I want them to fill there is another unit that can do it better and cheaper.
Now, what kind of conclusions can I draw?
Acts of Faith are too random to rely on.
Most Acts are minor perks, but at least they are a perk. It is not going to hurt spending points that would go to waste at the end of my turn anyway.
Repentia: I love these girls and really wish I could get more from them but the unreliable nature of Acts of Faith coupeled with their need to use Spirit of the Martyr to get back their cost makes it hard for me to justify taking them. Oh, I will anyway. They may be risky on paper but I have faith in them.