I am currently writing an adventure and one of the antagonists in it is a priest. But I am not really well versed in the holy part. So I am wondering if there's anything a priest can do vs a magi?
ALso is there anything a priest should have if they are going up against demons?
First thing you need to decide is if it is a holy man with faith, or a more mudane priest, politically inclined. It was customary for noble family that the second eldest son would enter the Church as the elder would inherit the land and the title, leaving nothing for the second son (and the third would go to the army to get honour for the family).
So these priest might not driven by faith but more by political clout, thus lacking faith.
Faith is arguably the most powerful supernatural power since nothing can oppose God's will in Mythic Europe setting.
So refer to p189 core rule for how Faith acts as Magic resistance and how it can be used in regular test, then page 202 onwards explain how the Divine influence can manifest, especially p 206 about Miracles which is the closest equivalent to magical effect, usually a lot more subtle.
Regardless of their personal Faith score, member of the clergy benefits from a divinely granted magical resistance according to their position ranging from 10 for archbishop to 25 for the pope (RoP:Divine p43).
Without granting obvious power to the priest, the characters should be keenly aware that as a priest he has the backing of one of the most powerful organisation in Mythic Europe: the Church. So even without any display of powers, he needs to be handled with care or consequences can be very serious ranging from Quaesitor's investigation for endangering the Order (high crime, death penalty, etc.) to the Church getting involve personally and raising a Crusade.
So the first power of the priest lays just in the institution he is representing.
Second, depending on his title (simple village priest, head abbot, bishop), he will have a certain political weight, leverage for various favors and legal power. He can also have access to large financial resources, again depending on his status in the hierarchy.
And thirdly, he might have access to Faith (through personal faith if is genuinely pious or through Holy relic granting him faith point (from 1 to 5)) granting him various protection and possible access to miracle (relics can have Divine might to triggers various powers).
Without access to RoP: Divine, refers to the core rulebook (as mention) and use miracles and god's will as it suits your story. A few guidelines to keep in mind, because He acts in mysterious ways, miracles are usually subtle and discreet, no big flashy pilum of holy light. Miracles should never kill humans since it deprives them from the opportunity to redeem themselves and seek salvation. In case of conflict against mages, one of the most powerful yet invisible effect is to grant immunity / high resistance to magic to the priest and his servants: mages are powerful, but once deprived of their magic, twenty foot soldiers can be a real threat.
If you grant immunity to magic, keep in mind that it does not have to be the equivalent of Parma, this holy resistance to magic could also prevent indirect magic to affect the target (Vilano's sling effect & cie).
When you flesh out your priest, consider his motives (how pious and holy are they ?) and how they would fit into God's greater scheme. Considering that one of the tenant of the faith is Free Will, direct intervention to railroad a certain conclusion is unlikely as it won't test the faith of the community.
For example, the priest has an objective that will interfere with the mages's own agenda. Three situation:
Against demons, priest have their protection granted by Faith, if he knows he will be facing demon, he might have relic and holy water granting him additional faith.
If you want to simulate spell without having access to RoP:Divine, consider that the priest can have access to spell-like effect equal to his Faith x 10 level, but because he is calling for Divine help, it takes usually times spent into meditation, prayers and devotion. As a rule of thumb, the less selfish, the longer the preparation the higher the chance of success. However, because God's acts in mysterious way, calling for miracles can expose the priest to retribution and possible punishment as well, especially if it is selfish, too frequent, etc...
Relics are highly effective against demons granting effects equivalent to Ring of Protection against demon and smiting demon with holy power (each round automatic damage equal to 10 x relic faith points, against demon's soak) within the proximity (loosely defined) of the relic.
RoP:Divine details a complete system for accessing various type of miracles, which are handled as spell-like effect (base level, range, duration), combined with different practices called Method. Honestly, they are designed with players playing holy characters in mind, so you don't really need it for NPC. It is a whole chapter and I have never used it since none of my players ever played a priest as companion or grogs.
Thank you so much for the help. I truly appreciate it. Would you mind taking a look at the priest and the demon and giving me some feedback on them? I have not yet fleshed out everything about the priest, but the idea is to have him be someone the church calls when they are dealing with demons. So an exorcist of sorts if you will.
The demon I made to be more of a nuisance and an indirect three, rather than the normal all-mighty demon archetype.
And again thank you so much.
First off that's a great and very comprehensive answer.
I don't rem,ember from which book, but I have in mind that demons cannot come within relic faith point value x 10 paces. So basic 1 FP relics (as granted by the minor virtue) keep demons at 10 paces.
First thing that strikes me is that the priest is missing the virtue for it, regardless of which one (the Church book has several, including one specifically for exorcists, which are below full priest IIRC). Without a virtue unlocking the academic/arcane abilities, he should not be able to buy any of the (realm) lore aside from student of the divine. Same goes for arts libs, theology and latin. Mind you, if this is an NPC whose sheet the players do not see, then it doesn't matter.
Shouldn't he also need a virtue for Intervention ? Maybe more of a personnal preference, but the priest is lacking in the skill that would make him a "normal" priest, like profession (preacher).
You are right, but I did not want to copy the full section regarding relics. They all have Scourging the Infernal and yes, demon cannot get close to relic if their demon might is lower than 10xRelic faith.
Regarding the characters, Medusa did a good work at highlighting some minor tweaks to do, but since it is an NPC no need to push to balance virtue and flaws.
Based on his skill sets, father Gabriel looks like an expert theologian/exorcist, poor in social skills. And it is an interesting archetype of character: he is an excellent exorcist, but because of his lack of social skills and investigative skills, he has to rely on other to do this part. He can be fooled or misled - he is smart, so he will eventually notice that something is not right, but he won't be able to find out by himself the truth. Such priest would not be looking for help to banish a demon as he is perfectly equipped, but he might need help or guidance to find Titivillus and not accuse innocents on the way.
I see two ways to play it (I am sure there are more, but these are my pitches):
Regarding Titvillus, I would increase his Guile skill to 5, and possibly Intrigue to 3. In fact, Titvillus is exactly the opposite of father Gabriel: he is a social animal, able to manipulate innocent people into doing his bidding without them realising it. So he can hide behind a wall of deception and mislead impatient investigator into sentencing the wrong target. However, as all demons, he is incapable of virtue like patience, so he will act a bit carelessly to try to precipitate the fall of the PCs which could lead to the discovery of his plot. Also, he has a very average intellect so he surely believe he is smarter than he is which would also be is downfall and he might try to make a clever-not-so-clever plan.
However, since he is not a fighting beast, the risk is that if he feels the heat, he will flee like a coward. So PCs and father Gabriel could be lead to a chase across several towns until they finally corner him.
The main way Titvillus would interact with PCs is by throwing at them clueless or mislead opponents: with his powers of corrupting texts, he could easily have letters sends to some PCs enemies, or at least neutral NPCs, but considering the PCs as hostile: maybe a Redcap get his letter to house Guernicus adulterated, a noble receive a warning that the PCs are planning to organise a peasant's revolt or support his cousin to take over his title. Even merchant guild could get involved: they received a massive order, but they are not paid (because the PCs never placed such order and the order was delivered somewhere else), then they send a team to repossess or retrieve was his their due.
I have another idea: what if Titvillus, fleeing father Gabriel, tries to hide in the covenant by possessing someone (probably a grog or servant, but a Companion might fit the bill too)? He might hope(1) that magi will be hostile to the church, and thus, protect him inadvertently. Should they slay the priest, all the better!
(1) Correction: Demons are incapable of hope. That's just his sooo clever plan! 
Things that would be useful to the Priest (other than Relic which has already been mentioned) include Sense Holiness and Unholiness or (probably but not necessarily) Divine aligned Second Sight.
For the demon, it's not clear to me exactly what type of sin / infernal plot it tends to try to invoke (although it looks to be something about misleading the holy / scholarly?) Detailing its obsession trait (i.e. what sin it invokes) would probably help, as would a blurb on its typical modus operandi.
Is the Ink weakness a Vulnerability or an Abhorrent Material? How does it interact with its power to corrupt texts?
That sounds like a very interesting idea. Even if it does not necessarily match what I envisioned. But gives food for thought for another adventure. =)
As a have taken a "real" demon and adopted it to Ars it might be that some things stick out of the typical mold for demons.
But as he is set now Titivillus is trying to disrupt humanity by corrupting their knowledge. Possibly even lead them to their downfall given enough time and enough people who cant see through his texts.
The weakness for ink is based on the "every demon needs a weakness" part in the RoP Infernal book. Cant remember exactly where tho. But the idea is that the weaker the deamon the more commonplace the thing. So I went for a bit of Irony with Ink being his weakness. If he is exposed to it it will damage them. Think +5 damage, and it would eventually kill him. And it is normal ink, although it has to be wet.
Regarding corrupting texts he either uses a proxy to write for him, or he is corrupting already written texts.
The spell is very similar to the power of Titivillus. But it serves a slightly different goal, as it is both a humorous thing in the adventure, and I would argue that it causes harm to the church in that people see what they believe is a drunk priest. It will cause them to ultimately lose faith in the church.
I don't know if this is accurate to your intentions, but ink is actually fairly expensive and laborious to create in the thirteenth century. It'd be present in relative abundance at, like, a university or monastery, and certainly well-off merchants and noble families would keep some around for writing letters, but most people (and markets!) wouldn't have any.
That might very well be the case, but in the circles where Titivillus is Ink is a fairly common occurrence.
Does the Divine powered Magic Resistance for priests prevent them from feeling the social effects of the Gift the way the Parma Magica does?
I can't recall if there is a ruling on this. But it would be interesting to see a Priest wondering just why his parishoners have suddenly decided to hate the harmless young child of the Innkeeper....
No. Magic Resistance by itself does not block the negative effects from the Gift.
Parma Magica does block them, but that is specific to Parma Magica.