Blessed with an ability to commune with the gods, priests are granted fantastical abilities to both heal and protect their allies, and bring harm to their enemies. Early on, though, they will have difficulty tackling the creatures of the Crypt or the Woodlands without aid from others; direct combat is their weakest ability, and it will be quite some time before they can learn some of their more powerful attack spells. They will, for a good long time, be dependent on the combat skills of others, but at the same time, they are adept at finding Fellowship with others. This is, perhaps, a Path most suited for Aislings that feel at ease in social situations.
Wisdom is the province of the Priest, and should be the primary focus of their development, both because it is needed to learn magickal Secrets, and because it helps them build mana. Intelligence is important, too, primarily for learning Skills, and they will need to develop their physical attributes as well, both to learn certain Secrets, and to keep them from being completely vulnerable. Within their first couple of insights, they'll want to make sure they raise their abilities to the point that they can learn beag ioc, perhaps the most important Secret for a young priest to know.
Fortunately, your students don't have to travel far to purchase their first-circle armor: both the Cowl, for the males, and the Gorget Gown, for the females, are available from Brody, the Mileth Tailor. When they reach the second circle, male priests can purchase a Galuchat Coat from Huberto, the Rucesion Tailor, while females need to go to the top floor of Loures and speak to Jean to purchase their Mystic Gown.
As for other implements, I would recommend that first circle priests use a Stick or Dirk as their primary weapon, as these drain the least amount of mana; hopefully, they won't be engaging in much direct combat, anyway. For the same reason, it is recommended that they avoid using various mana-draining secondary pieces of armor, such as gauntlets and greaves.
To use one of their most useful spells, beothaichdeum, used to prepare the potion of the same name, priests will need a large supply of fruit, specifically one bunch of grapes and two bunches of cherries for every three potions they hope to produce. Various Grocers about the realm sell these fruits, or if they wish, they can travel to Suomi, where they can harvest the fruits themselves.
Undoubtedly, the most important Secret for a first-circle priest to learn is beag ioc, taught by Devlin, the Fae Priestess of Mileth. With this spell, a priest will be able to use his mana to heal other Aislings, the primary function of a priest in any hunting party. Beyond that, the spell is a prerequisite for beothaichdeum, a very useful and important Secret. Devlin can teach other spells to your Student, as well, and I would recommend learning both armchahd and beannich as soon as possible; other than the coins to purchase the spells, Devlin also requires Raw Wax to learn armchahd. Priests I have spoken to fall into two camps when it comes to learning the fein or "self" spells: armchahd fein and beag ioc fein, both of which only affect the priest himself. There are those that believe that learning them is a waste of time, money, and experience, since their effects are duplicated by spells that work on anyone, and because casting them when you could cast their counterpart robs the Student of important skill levels in each of them. On the other hand, there are those that believe that learning them is worthwhile, since they are quicker than their counterparts, and consume less mana, and allow the priest to protect himself without sacrificing his efforts to aid his compatriots. I fall in line with the former, thinking that a Student would be better off learning, and mastering, spells that can be used on anyone, but I feel that this is a personal decision and is ultimately up to the Student.
After raising their beag ioc level to at least ten, your Student can learn the beothaichdeum spell from the Fae Priestess of Piet. As stated above, the student must have one bunch of grapes and two bunches of cherries for each three potions they hope to produce. They then cast the spell, which blesses the ground underneath them. By walking back and forth upon this now-sacred ground, your student can remember the Faerie Lore, and transform the fruits into Beothaich Deum ... if they're successful. If they fail to remember the lore, the fruits will either sour, turning into Wine, or they will rot. I encourage my Students to save the Wine, as one day they will be able to learn the skill Herbal Lore, at which time they will need a good deal of Wine. Even the rotten fruit can serve a purpose: they should take it to Cian to see if he needs some; even if he doesn't, he will usually pay them for it.
As they gain insight, Students should be encouraged to speak with other Mundanes about new skills they can learn, as well. Some Rogue Trainers will be able to teach them Lore skills, and Dark Wizards can offer offensive spells.
Unfortunately, Priests are simply not powerful fighters, and it is unlikely that they ever will be. When they must hunt alone, they should cast protective spells upon themselves before entering battle, and heal themselves immediately afterwards. But a priest is most effective when hunting with others, when they can stay away from the front lines and concentrate on protecting and healing their allies. Later on, much later on, they will be able to learn some truly powerful offensive spells, but until that time they will be reliant upon others to hunt with them. Fortunately, priests usually do not have a problem finding hunting partners.
One technique I've picked up from the priests I've hunted with is for the priest to cast protective spells on each of the party members, then on himself. While the priest, hopefully, won't be exposed to the front lines, the spell can serve as a sort of "timer", allowing him to know when its effects on his allies have worn off. Oftentimes, a Warrior or Monk doesn't have time in the midst of a battle to inform the priest that armchahd or fas deiras has worn off, and in this way the priest can cast it on them again.
Priests, by their very nature, are perhaps the most social of Paths. They will often need Assistance from others to pray or perform consecrations, and as with Monks, brewing herbal potions is much more effective with an Assistant.
But more than this, when they join a Fellowship, they become a member of a community, and can participate in this community as much, or indeed, as little, as they wish. It is recommended that priests study the gods soon after receiving their oath, so they can decide which Fellowship best suits them. Do not, however, attempt to influence your Student's decision; merely answer any questions they may have about the gods in an impartial manner, and let them choose their own Fellowship.
Unlike other Aislings, those of the Priest's Path can initiate themselves into the worship of a god at his temple; they do not need another priest to initiate them. Joining a Fellowship will open up a whole new world to a Priest, as they begin taking part in their respective community, relying on them for aid and counsel, giving aid and counsel when they can, and using their Faith to perform miracles from the gods.
Faith is very important to a priest, so the sooner they join their Fellowship, the sooner they can begin building it. Your student can build Faith in three ways, at first: by Giving Thanks, attending Masses, and by performing a Geas for another priest. To Give Thanks, a priest must pray to their god, or to one of their god's allies; doing so builds Faith, but doing so in Fellowship with another Aisling builds it even more. A priest, or any worshipper, for that matter, can attend the Masses of his own god, and those of his god's allies, once per double-moon; Mass schedules are listed in the Events of Temuair, and are usually posted on the Message Board outside each Temple. To perform a Geas, a priest must accomplish some task in service to his god; once he has completed the task, the priest that bestowed the Geas upon him can complete it, granting your Student a great deal of faith, and insight.
With enough Faith, a priest can Ascend to the Priesthood of his god by speaking of Fellowship to the attendant within his god's temple. I used to think that the priest had to have been of at least the eleventh insight to do so, but have recently learned that some priests are able to Ascend to the Priesthood at earlier insights, depending on their level of Faith.
Once your Student has Ascended to the Priesthood, he can also build faith by Desecrating items Consecrated by his god's enemies. Doing so requires Faith, but returns an even greater amount of Faith. I'm told that doing so with an Assistant requires less Faith in order to perform the Desecration, and returns more Faith than it would otherwise.
At his eleventh insight, a priest may also Consecrate items to his god. Doing so grants the wearer of the item certain benefits; refer to Tryston's Gifts of the Gods for details. Again, Consecrating with an Assistant requires less Faith than doing so alone. Contrary to popular opinion, any worshipper of the priest's god, or that god's allies, may Assist with a desecration or consecration, not just other priests.
Another way for priests to gain Faith is by sacrificing at the Altar in Mileth. Priests are expected to make at least one sacrifice per Temuairian day ((3 hours)), and while doing so is said to increase one's Faith, it is said that doing so more often will not only not increase one's Faith, but may very well draw the wrath of the gods for idolatry.
Eventually, your Student may take a more active role in his church, perhaps even Assisting at a mass, or giving a sermon. If and when this happens, I would recommend you attend that mass, even if you don't worship the same god; this is your Student's moment to shine, and if you have been a good Mentor, I'm sure he'll want you to be there to share it with him.