The big deal: Faith and Until the Light Takes Us
A novel and deeply grimdark take on Acts of Faith: if I read right, you only get AOF after an independent character or squad is wiped out, but then EVERY unit in the army gets its AOF at once? Very powerful, and potentially abusable if players sacrifice a cheap IC (eg Palatine) or squad (Frateris) in suicidal charges every turn. Maybe you don't do that, but ask a devious, ruthless power-gamer to playtest and see what havoc they wreak....
Potential abuse aside, when combined with the rules for half-strength squads and squads down to their last model, it's a beautifully bleak vision of Faith.

Sister Pronatus infiltrators are cool, but they should really be elites (compare my Vigils, whose AOF also resembles your Celestians').

Frateris Militia: Essential to an expanded Sororitas list, but what does it mean that "only" the Preacher has Stubborn? Doesn't a single model convey that special rule to the entire squad?

Retributors: holy crap, so I can have a squad with 10 models and 9 heavy bolters/flamers/multi-meltas

Crucifier: all in favor of Sororitas super-heavies, too, but maybe this is a trifle undercosted for a twin-linked Titan-scale weapon?

Some typos and awkward wording in the Act of Faith section

Litanies of Faith: If you say Leadership tests, that automatically includes Morale and Pinning, as well as Acts of Faith, Fear tests, Guard Orders tests, etc.

Phial: Why bother noting that the Canoness loses Stubborn? Seems like wasted words.

What's a Hellfire Rifle? I know what it does, but what is it?

You've included a lot of stats lifted from the main rulebook. It would probably be more efficient (and save pages, making it cheaper to print) to simply put a note at the top of the wargear section saying "the rules for the following weapons are in the Warhamer 40,000 rulebook:"

Sacred Banner: No bonus attacks makes Miko a sad kitty. :(

Imagifier: what is it? An Imagifier is a Sister who carries a Simulacrum... so this entry confuses me a bit, especially since its effects seem rather strange.

Heh, I'd forgoten that Sabbat predated the Adepta Sororitas . This does raise an issue where you state that she had a command of Sisters Militant, however.

... Which Canoness became a rogue trader? Oo That sounds utterly wrong.

You've put the Seraphim fluff in twice and wrong. >>

The Seraphim, Dominion and Retributor sections - you appear to have suddenly switched from talking fluff to talking about game mechanics? It's a bit jarring

Living Saint - Is she a generic Living Saint, or is she Celestine? From the gear and stats, I'd say Celestine, but...

Why is the Canoness 75 points? She's over-costed compared to HQs with similar stat lines and wargear at 65... Also, I'd separate out her Rage rule from the Act of Faith - the Palatine has the same Act of Faith but without the Rage part, which is obvious enough on paper but will easily get lost in the mental shuffle mid-game

Sororitas Command Squad / Celestian Bodyguard. Double the price for half the fun? I don't get it, what makes this unit worth taking? Why are the Celestians worth twenty-five points each for a stat line worse than a Vanguard Veteran's?

Also, if you're giving them Jump Pack abilities, you should probably let them swap their bolter for a bolt pistol for free.

Wait, wait, wait, so standard Celestians can take five special weapons but the SCS can only take one?! But the standard Celestians are 5 ppm cheaper?!

Repentia with double power mauls? Cute, but weird. Also, why would you? At I3, there are far better units in the codex to hunt Guardsmen with.

Also, why no Repressors for the Repentia? Seems like a vehicle designed to ram into the side of a building and then let its cargo out to wreck face is perfect for them to me.

Why is the Penitent Engine only S4? It's bigger than a Dreadnought!

Arco-flagellant has Repentia statline.

if Light of the Emperor is Preferred Enemy (Everything), why is The Passion also Preferred Enemy (Everything)? Why not give the Palatine Light of the Emperor and keep The Passion for the Canoness-only Rage-inducing version?

Sacred feth that's a lot of special and heavy weapons in a line squad!

Also, if Flamers are free, then Storm Bolters should certainly be less than 5 points for a mere extra shot at long range or when charging! Also, why no Multi-meltas?

but why doesn't the Pronatus gain the benefits of Light of the Emperor?

Why shotguns? No Sister ever uses a shotgun. The Infiltrating Troops is balanced out by the fact they can never take a flamer or melta, so that's actually.. okay

You should probably change the wording of Demagoguery to state that a unit fleeing as a result of losing its preacher can regroup on the leadership of an independent character who joins the squad while it's fleeing.

Back down to 2 fire points on the Repressor? Even Forge World realised that was a mistake and put them back up to 6 in IA2:Se

Seraphim are very nice, especially with always getting Shred when they deep strike.

Aww, back down to 2 per 5 Dominion special weapons? This just makes your Celestian squad look even more ridiculous. Even the white dwarf codex which introduced 2 per 5 didn't get played that way by the guy who wrote it - he used 4 per squad in the battle report in the same issue as the 2 per 5 codex!

Immolator profile is repeated for some reason.

Exorcist. More reliable shot count for reduced AP? No thanks. Exorcists are only really worth it because AP1 makes them good against Terminators and vehicles, otherwise S8 is just... completely lackluster.

Retributors are overcosted per model. Rending is not worth more than Ignores Cover.., wait, they come with Heavy Bolters standard. They upgrade to Heavy Flamers Free. Holy gak. 15 ppm? Try 20.

... wait, +10 points for Multi-meltas, the worst heavy weapon option of the lot? Are you sure you've got that the right way round?

BS4 Avengers. Nice, but why take away Strafing Run? Effectively all you've done is turn it into a better interceptor, which is not what the Avenger is meant to do.

I'm not a big fan of the Acts of Faith system. There's no way to use it smartly, you'll either have it that turn or you won't for your entire army. Also it encourages Killing off a unit a turn so that the rest of your army can be effective. It also scales horribly, in larger games you're almost guaranteed to have Acts of Faith active, because your opponent will be able to focus fire with multiple units to take out one of your many squads. Small games might not see any Acts of Faith until turn 4 or later.

Speaking of unnecessary, Bionics are something that should remain in 3rd edition. Like purity seals it limits modeling opportunity, and is too minute of a deal to make it's own wargear. Even Iron Warrior Chaos Space Marines don't get Bionics so why should Sisters?