In Grave Peril, when Michael seems surprised that Harry already knows Father Forthill, Harry mentions that he once asked Forthill to bless a barrel of holy water. He then says "Talk about your surprised ghouls".
See OW58 (the text for Ghouls, not the statblock):
Weaknesses: Affected by holy water.
Unwavering inability to control their appetites
for long. Most are kind of dumb.
Regarding Ghouls and thresholds, I infer the following from the rules (though there are no specific rules for ghouls and thresholds in particular):
* A threshold acts as a block against supernatural creatures. As this affects Wizards, I imagine it would affect Ghouls, as well. (Note that this makes entering more difficult, but not impossible.)
* A threshold suppresses some supernatural abilities. All affected supernatural abilities lose shifts of effectiveness. Since the books indicate that thresholds provide defense against ghouls, I would assume that their supernatural abilities would be affected. How much? Hard to say, though as a first guess I would say that two points of threshold would remove Inhuman Strength and Speed and reduce Supernatural Recovery to Inhuman; four points of threshold would eliminate Recovery, too. Most likely ghouls would be unable to feed while affected, though they would still be subjected to the negative effects of hunger, etc. Oh, and any threshold at all would likely nix Human Guise for the duration.
* Some creatures (creatures of spirit) are entirely supernatural in nature and can't cross thresholds at all. I don't think ghouls would qualify for this, however, as they are nominally semi-human.
Note that holy places have a default threshold strength of 3, but could be higher, especially if inhabited by a person of True Faith (Bless This House would likely increase this to 5).
That said, it isn't entirely clear to me whether you intended your version of Mother Theresa to be a truly holy person who was also a ghoul, or a evil ghoul impersonating a holy person. Your references to the Sabbat lead me to believe the latter. In this case, consider that while holy places have thresholds ... not all building that are used as a 'church' are necessarily automatically a holy place. An evil ghoul might work out of a building that is 'impersonating' a church without actually being one, and therefore without having a threshold. So the evil Mother Theresa would have no threshold-related problems. However, the 'church' would also likely have some fairly foul aspects associated with it (Mother Theresa's Cathedral, Where The Other White Meat Is Always Fresh!), which would likely be palpable in some ways to even mere mortals. And, of course, she'd have severe trouble entering actual holy places.
Of course, these are just my takes on the question.
