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:
- Not divine intervention: if the mages want to use mind magic to change the priest' idea or force him to agree and sign document that would help the mage to the detriment of the church and the community, let them have it. The mages are letting them open to the sin of sloth (easy, comfortable solution), and might later one face consequence as by preventing the priest goal, maybe demonic infestation will happen, famine, or the priest will get vision that he has been tricked someways and escalate the problem through some connection to Quaesitoris that will start to investigate. The underlying theme of such adventure could be "Does power corrupt really?", it could be God's test if mages can use their magic for good, how selfish can there be or are they utterly damned ?
- Partial divine intervention: the priest will be immuned to mentem spell and other coercion, but if the mages are able to convince him by the use of mundane skills, he will hold his end of whatever bargain would be reached. This is testing both the priest skills and the mages skills and morality. It is another way to test if magic tend to corrupt mage: would the mages have handled the situation differently if magic could have sway the priest ? Once deprieved of magic, are the mages more "reasonable", are they willing to hear the plea of the less fortunate onces ?
- Full divine intervention: the priest has a invisible angel escorting him and making him aware when mages use magic, for which ends and protect him from it. This is more confrontational: for some reason, God's decided that the priest should be fully aware of the mages' abilities and uses of it. If the mages refrains from using their powers, without knowing that he is under divine protection, it reflects well on them, showing that at least, they respect the Church. If they spontaneously offer helps or assistance, they could possibly be considered as assets, and why not allies for the times to come. If they try to resort to magic to bend the priest's will, then they are not part of the solution, they are part of the problem. There might not be any immediate consequences, but in a close future, something will trigger a reaction and they won't be spared.
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.