Is there anyhting however in the books to prove that this power comes or doesn't come from the Emperor as opposed to the faith itself? As in, would a sister that fell into heresy and actually stopped worshiping the Emperor lose her powers?
Depends on which books you mean. As far as GW's own material goes:"The Daughters of the Emperor studied ancient arts of war, using a taxing learning process to clear their minds of all worldly considerations, honing their skills over their entire lives."
- 2E C:SoB"For millennia, the Sisters have practiced their unique method of war, combining combat doctrine and prayer which enables them to accomplish feats upon the battlefield that appear miraculous to the unschooled."
- 3E C:WH "The perfervid, unquestioning nature of this faith is a potent weapon indeed, manifesting as divine inspiration that drives the Sororitas to unprecedented feats of martial prowess."
- 6E C:SoB
If it would come from the Emperor or even just the Warp, it would probably count as a psychic power - which would be contradicted by the 3E Shield of Faith (which auto-deflected even beneficial
psychic abilities from their allies) as well as the rules in GW's Inquisitors game, which stated that AoF are "not counted as psychic powers and so may not be nullified", as well as that "no Adepta Sororitas character will ever have psychic powers of any sort".
Or, in short, to me it seems like the original idea was a sort of "mind over matter" approach, reflecting a mixture of intense training and uncompromising, zealous dedication to the cause which sees the warrior banish all fears from her mind, ignore dreadful injuries and pain (possibly simply delaying her death/unconsciousness) or summon preternatural strength. We have accounts for this sort of stuff happening in real life, from soldiers ignoring multiple bullet injuries to some frail girl lifting a 5600 pound jeep off her father's leg. The Sisters of Battle are quite simply an army that has this as a part of their theme.
This also explains various TT rules such as Acts of Faith depending on Leadership scores (squad leader inspiring her Sisters) or the death of their comrades-in-arms (increasing the "zeal level" by evoking a thirst for vengeance).
Obviously, there are many other interpretations in existence once you look at the licensed material, though. In Dawn of War as well as FFG's Blood of Martyrs supplement, it is rather obviously space magic, with effects that are much flashier than anything from GW's tabletop - which should probably make it difficult for the Space Marines to continue saying "nah, he's not a god" when the girl next to them starts shooting lasers out of her eyes after saying a prayer. Either that, or they kill her for unsanctioned psychic activities.
In the Inquisitor's Handbook, a book still written by the original Black Industries team, a Sister would also lose her Faith powers if her Corruption score ever rose above 10, which kind of fits to the original material: no Acts of Faith
without actually being faith
ful. Without religion to serve as a catalyst for their willpower, they'd be nothing but very well trained soldiers. This is not to say that, in theory
, they could not start following a different religion with the same zeal (Chaos?) or even just believe in a worldly cause (Imperial nationalism?) strong enough that it'd have the same effect - but aside from the very low chance of one of them falling in the first place, the character would have to survive her "crisis of faith", and any new belief she adopts likely would not be as strong as the first one (if worldly it misses the conviction about supernatural aid from the Emperor, and if religious it would not be as unquestioning considering she'd already forsaken her first faith).
I believe the loss of Pure Faith due to Corruption was not part of FFG's remake of the Sororitas as published in the Blood of Martyrs supplement, though, so that characters created using the rules from this book would indeed keep their powers if they fall.
Edited by Lynata, 02 December 2013 - 12:13 PM.