Rare Ability and how to use it

[MesmerOfLife]

I'm creating a Chorist mage with all of this Christian stuff like crests and the Bible.
And I thought that it's good to take Theology or Belief Systems for my pure quintessence of christianity.
And then I realized that I can't come up with any use of these Abilities.
With others it's all easy. If you had PhD in chemistry but your character has only 1 dot in Science, you pray while rolling Intellect+Science when your character is starting a chemical reaction. Or you don't know physics at all, but your Virtual Adept uses quantum physics for teleportation and you roll 7-8 dices to reduce the difficulty of using Correspondence.
But how to use Theology? Roll Intellegence+Theology to say that Jesus died for our sins? Belief Systems entry said that character with this Knowledge can use the Reality Zones and make his magic seem coincedental. But how? Cause the Reality Zones entrance said "no rolls, you must roleplay it". So it's useless. Even Carousing seems more usefull to "Drink a barrel of beer on a bet and not vomit".
So it's a seems right to represent my mage knowledge by dots in Theology, but I can't find a practical use of it.
And the same question for a lot of others secondary abilities.

[Bluecho]

First, some Abilities are taken because they are logical for your character to have them. If your character went to Seminary, Theology is the sort of stuff they would have been taught. Not all Abilities are meant to be immediately or commonly useful. They are just meant to reflect Real Life and who your character is.

Second, M20 has the Minimum Abilities rule (though it's optional), which maintains that a character might need a minimum score in this or that Ability to even cast certain levels of Effects. Even if the Sphere dots listed on your sheet say you can cast a Life 3 healing spell, if your character wants to pray to accomplish it, but knows nothing about Theology, they might not know where to start asking for such a miracle. Let alone more esoteric supernatural Effects. It's the same reason why the Virtual Adept would NOT be able to teleport using quantum physics knowledge he does not possess. How does he even know what he's doing? This rule, obviously, is up to the Storyteller to decide, but it's a pretty safe bet that if the character uses it as part of her Focus, she ought to have its mundane Ability component on her sheet.

Third, Theology can be very good for research purposes (when the character tries to develop or discover a Rote), for dealing with the Umbra and its spirits (mostly angels and demons, though Esoterica can also be used for this), and for deciphering mysteries that have a theological bent (like when dealing with cultists). The character might also be called upon to roll against their Theology when trying to convert someone (important for Mages, who need to change people's beliefs in order to modify the Consensus), when their Mentor or Avatar tries to impart knowledge, or during or in the wake of a Seeking (especially if the Seeking failed, and they need to puzzle out why; for a Chorister, the Seeking will likely have a theological bent).

Fourth, Belief Systems is a useful Ability in the Ascension War as a whole, since it can help you understand the Consensus and its constituent parts. The aforementioned bit about converting someone may be made easier if you can grasp the nature of a person's belief, especially if they are different from your character's own. An argument for why a person should convert falls flat if you, yourself, don't actually understand what another person believes and why.

Five, if the book says no roll is possible for Reality Zones, I'm disinclined to believe that's accurate. You just need to understand what Reality Zones ARE. They are areas where the beliefs of those who inhabit it are strong enough to deviate meaningfully from the nebulous "core Consensus", which affects what is Coincidental and what is Vulgar. As such, you should be able to roll against Belief Systems to grasp what the Consensus is in an area. The Storyteller can be made to tell you what is Coincidental and what is Vulgar, which you can use to tailor your Effects accordingly.

Six, Belief Systems is probably good to have if your character is trying to adapt a Rote from another Focus to her own. If your character can understand why another character believes their Rote can work, it will be easier to figure out how to make it work in her own system. This can be a useful skill in a cross-Tradition game, where knowledge can be shared but not necessarily understood or applied. It's also useful for when characters need to work together on a single Effect, but have different Paradigms. If you want to collaborate on a Ritual together, you must both agree on a Ritual structure, or you won't get very far.

[Aleph]

With others it's all easy. If you had PhD in chemistry but your character has only 1 dot in Science, you pray while rolling Intellect+Science when your character is starting a chemical reaction. Or you don't know physics at all, but your Virtual Adept uses quantum physics for teleportation and you roll 7-8 dices to reduce the difficulty of using Correspondence.
But how to use Theology? Roll Intellegence+Theology to say that Jesus died for our sins? Belief Systems entry said that character with this Knowledge can use the Reality Zones and make his magic seem coincedental. But how? Cause the Reality Zones entrance said "no rolls, you must roleplay it". So it's useless. Even Carousing seems more usefull to "Drink a barrel of beer on a bet and not vomit".

In the vein of your examples:

Roll Intelligence + Theology to lower the difficulty of any christian ritual from milacre healing to smiting your foes, as you recite appropriate versicles of the bible, call down the appropiate saints or summon the correct angels (depending on your sect) to call down the milacre.

Not all angels are equally related to the wrath of god and surely you have seen preachers: When they preach they do recite versicles that have the appropiate connotations to argument their speech and cause the desired emotions and such

As per Bluecho examples, "Belief System" it's strictly more useful to day to day situations because it's more general and because Belief it's a big deal arround mages, but Theology may be better if you deal with angels that may get offended if you can't recite the bible (or with christians, that may respect you more if you can). And it makes sense for your character, even if the ST doesn't enforce it's use