Peacekeeper_b said:

When talking about player characters, you aren't talking about a typical member of whichever group they belonged to. Instead, you are talking about an individual who stands out from the group enough that they get recruited into the Inquisition and manage to survive the work they are given.

So while it may be hard to believe that all Sisters of Battle have pure faith, it is easier to believe that all Sisters that work directly for an Inquisitor have it. It's probably a case that, if an individual Sister doesn't have pure faith, she won't get recruited into an acolyte team.

Bilateralrope said:

Well, not even Inquisitors have the means to accurately measure a Sister's faith, and many Inquisitors may not bother, instead being interested solely in the loyalty (to the Creed), knowledge, skills and/or combat prowess a Sororitas character could offer. If there really were "Impurely Faithful" Sororitas, they too should be playable.

But there aren't. I hold the belief that Pure Faith is quite simply the means to demonstrate the minimal level of zeal a character from this organization would possess - that which sets the Sororitas apart from the normal clergy, the Frateris Militia and Redemptionist cults in the fluff. BoM goes on to offer actual Faith Talents as optional upgrades for further individual character progression, and though I believe they become available too soon, I do think that the basic principle behind this breakdown is sound and, for once, in line with what we know so far from studio canon.

In short, not all Sisters' connection to their faith may be so extremely strong as to actually evoke minor miracles out of their beliefs, but their lifestyle and indoctrination do result in the semi-unique specialty that is Pure Faith.

As always, of course, these thoughts are just my two Thrones on the subject.