I think I'll offer my answer the OP question this way:
The True Faith powers as written in the book represent True Faith in the Almighty, which covers the faiths of Christianity, Judaism, and Islam. (Not all aspects of those worldy religeons, but those aspects that truly (or Truly) adhere to the wishes of the Almighty, which because it's a question of Faith may end up being a judgment call by the GM and/or table. It might also be ruled that followers of some other religeons are actually followers of the Almighty, though by a different name. Again, a judgement call. Likewise, those who do 'the right thing' as an expression of personal belief without expressly being a part of a religeon might even qualify (Sanya would be a great argument for this).
In all of these cases, the characters are qualifying because they are genuinely following the wishes of the Almighty (who is definitively real in the Dresdenverse, OOC arguments aside), knowingly or not. I do not believe that characters who a "true adherents" to religeons that contradict the will of the Almighty would qualify for True Faith powers.
That said, I could easily see justification for an alternate set of True Faith-like powers that *could* be used by True Adherants to other sufficiently powerful entities. Some of these powers might be carbon copies of True Faith powers, while others might have a very different flavor. For example, I think that by nature, powers granted by an evil power would tend toward aggression, rather than defense (so "Bless This House" might be replaced by "Aura of Defilement" which grants a bonus (+1 or +2) to attempts to destroy thresholds).