Ok I've mostly finished the game on Hard mode with a Priest. This is the build I used; the info would have been helpful to me when I started, so I hope it is helpful to others as well.

Attributes: Primary attributes are Intelligence and Speed. Intelligence should be taken to whatever is needed to obtain magister level in the school you are using. Speed is important because it enables you to escape enemy attacks. With enough speed you do not need to invest in constitution or defensive skills because you will not be taking hits. Having a high speed also makes the game much more convenient as it reduces the time it takes to wander the town/explore the map.

Act 2 Attribute note: You will hit the maximum 50 speed and need to start investing in constitution. The hit points will be needed to survive some of the ranged magic using monsters that start appearing in act2. You will also need to eventually take Strength to 36 to make full use of Holy Aura.

Act 3 Attribute note: You will eventually be able to max dex and str to 50, but I think the better way to go is use those points on Intelligence to get a bigger mana pool. Some of the spells at Magister level take a lot of mana to cast.

Skills: Focus on 1 school of magic and pump it up as high as possible. Secondary skill is Wisdom, which is essential for casting spells. Other skill points should go into secondary magic schools, though this is only needed for convenience (spells like Levitate). I did not use any forbidden magic schools as I'm trying to play a pure 'good guy'.

Act 2 skills note: Now that offense is taken care of, its time to focus on defense. Take Holy Aura to at least 16, you can finally put 6 into armour and start wearing chain. Choose one school of magic and take it to 11, leaving the others at 6.

Act 3 skills note: I pumped up a few more magic schools to level 16 and started maxing Flagellism for the extra defense. As I maxed out the magic schools, I was able to start discarding my +1 to magic skills items.

Spells: I spammed 1 attack spell (whichever did the most damage at the time), used 1 buff (Bless) to make my spells hit harder. Very rarely I would use Heal, Light, Crucifix to paralyze stronger enemies, fireblade to give the companions a bit of extra punch and I never once had to use Raise Dead - I considered these mere convenience skills and not required to complete the challenges. The only others worth mentioning are the defensive/resist/protection spells which I only used when I was fighting powerful magic user boss type monsters.
*** The spell with the highest damage is Locust Swarm, which suggests Magic of Miracles is the way to go (thanks to gregn30)

Act 2 spells note: Not much will change here, except you might want to start making more use of the defensive buffs to reduce damage from the spellcasting enemies.

Act 3 spells note: This is where you start getting more powerful attack spells. The one Divine spell that can kill undead in 1 shot is especially great since you encounter so many of them. Powerful spells cost a lot of mana, so adjust the potion ratio accordingly.

Equipment/Items: Some shops give better buy/sell prices than others (sometimes after helping with quests, shops will give better prices as well) - find out which is which and maximize your gold when buying and selling loot. Try to get Magister + magic skill items with + to speed attribute (or all attributes if you have the patience and money) from the shops. The other useful one is + stamina regen. When I started out, I had all + stamina regen items, then replaced them with + magic skill w/ speed items when I had the money. I used leather+shortbow since I had no skills in weapons or armour, and that worked just fine for me.

Act 2 equipment note: I dropped the bow and went with weapon+shield so I could get more item bonuses. The damage from the arrows starts to become less noticeable. Wear chainmail after obtaining armour skill and try to get some +AC vs ranged attacks. Try to keep around 12,000gp in your pocket at all times, just in case you see a +to all magic skills / +to all attributes item. Otherwise, spend all money on magic boxes.

Act 3 equipment note: Try to get +to all attributes items on every item. You should only need +4 to magical skills in total to max everything to 20. I would recommend 1 buff to stamina regen as well.

Yellow equipment: some unique items are given to you as quest rewards, and they have a yellow colour. Whenever they are given after a conversation (like the judge ring) you can re-roll their stats by saving and re-doing the conversation. Since they can have more bonuses than any other colour, they have the potential to be the best items in the game.

Seals: do not buy Seals. Use the ones you find to open locked doors and containers.

Magical Boxes: Destroying a magical box and dismissing the genie gets you a chance for a skill or attribute point. I saved and reloaded and used them exclusively for skill points. Spend all of your extra gold on these. They are expensive, so make sure you maximize your return by shopping at a cheap store!

Act 3 Magical Boxes note: Use these for both skill and attribute points. These are such a rich source of points that it cannot be overstated how important it is to make proper use of magical boxes throughout the game.

Potions: Use 1 full inventory tab for potions, 1/3 of each color. The priest will only ever need the blue ones. The red and yellows will keep the companions (meat shields) fueled up and alive. Make sure potions are kept topped up every time you re-enter the town. I have never had a companion die with this setup. In the beginning, you don't need to fill the full inventory tab, and use the gambling to generate enough money to pay for pots if you are low on cash.

Act 3 potion note: I eventually ended up using 2 full inventory tabs for potions, with 1 entire tab dedicated to yellow.

Devilroot Potions: Since these can take you beyond the natural maximum in an attribute, I recommend saving them until you hit 50 in speed, which is the max for the priest. Then drink all speed pots to maximize your speed! The only downside to this is you would have to wait for Act 2 to see any benefits from the Devilroot potions, so its up to you where you want to have an easier time, Act 1 or Act 2. (personally I think Act 1 is easy enough)

Tactics: Have companions take enemy aggro and spam spells from a safe spot. For maximum damage, cast a spell, attack with arrow while spell is cooling down and then cast a spell again. In dire situations, just use your speed to run from enemies while spamming spells back at them while they chase. Sometimes you will get 'rushed' after a dialogue (like Herbertius) - for those situations, again just use your speed to run back until they hit your meat shields, heal up and then smash with spells. If you run out of potions, stop whatever you are doing and return to the town to fill up. Potions are absolutely essential to keep the meat shields alive and spells slinging. I found the mana regen items too slow and the mana regen skill mediocre, much more worth it to just use mana pots than invest in those.

With this setup, I pretty much waltzed through Act 1 on Hard mode. Acts 2 and 3 were a little tougher but ultimately not that challenging either.

Final thoughts on hard mode: I found that most of the monsters seemed to have way too many hit points and there were way too many of them as well. (And too many wandering monsters on top of that) Monsters don't tend to have special abilities that do anything interesting or lead to varied combat scenarios, so killing them becomes a trivial, boring, repetitive chore. I have no experience with normal mode but if it lowers the hit points of the monsters or has fewer mobs, it is probably the superior mode, if only to avoid several hours of very boring grinding.

Final thoughts on the game: There are alot of problems with this game, as have been noted by many who have played it: The randomizing loot system is annoying, plot themes are repetitive, characters are one dimensional, conversations are linear, the detective work to find the heretics is basic and simple, monsters all seem the same, combat is non-tactical and tedious, the way they handle bashing locks is just insane, and everything is based too much on potions.
Having said all that, the game does offer different classes, lots of skills and spells, stats, lots of magical items - and you get to torture heretics and burn them at the stake, which is a hell of a gimmick for a game.
I would recommend this only for the hardcore, and only if you like the gimmick.

Nice write up. Now you've done it though... I guess I'm going to reroll now. :P

Aaaand even when I'm looking at all the things Inquisitor does wrong, this here is what I love about the game. It doesn't hold your hand. At all. And discovering how everything works is actually pretty damn satisfying, especially today.

Good info!

I have restarted also on HARD with a priest, and I find it EASIER than on EASY, because of the better items and better companion stats.

I took my time and re-loaded until I got +15 to ALL Attributes for the two uniques you receive early on in the starting town: <span class="bold">Boots Of Saint Baudolino</span> and <span class="bold">Prosecutor Ring</span>. +29 to all attributes is a VERY good start-up :-) compared to my pity-full first time Palladin (IMHO) ...

- Do you have a CHAR screen-shot for your avatar at the end of Act1 ?
- What stats matter most for an item (+experience, +damage reduction, etc).

Hah see I didnt know you could re-roll yellows...

For stats on items, the big thing to remember is to stack whatever you are trying to get, so if you want +experience, get a bunch of +experience items and stack it as high as you can, otherwise you wont notice the difference.

I used +stamina regen items at first because the spells start out so wimpy that ranged attacks are actually more effective, and also since your stamina reserve is so small it runs out way too fast. (also also I was trying out the mana regen skill - which requires massive stamina but turned out to be mostly useless)

Once I started getting more powerful (and more rich!), I switched over to the Magister +1 to magic skill items because you really need to pump those magic skills in order to get decent damage/effectiveness. The + to speed is just an added bonus.

Now that I'm in Act 2 I have about 40,000 gold saved and I'm going to blow it all on Magister + to magic skills / + to ALL attribute items :)

The thing about this build is that you will mostly not be taking hits, so you just dont need damage reduction, resistance or AC protection... its all about SPEED! Honestly most of the other bonuses on items seemed pretty useless to me.

Wow, you must have strong will. I try playing priest on hard but this is so goddamn boring. Currently lv 10 and best spell hits for 15-30 (trolls, ogres) up to 40 (spiders). I don't play with companions. I think i won't even end first act because this dmg i so sad, especially when i have paladin and thief on normal difficulty futher in story. Or i will spend all money on seals and somehow i'll get lvl 15 and better spells. Seriously, such weak character not counting seals is no fun at all.

I think companions are pretty much essential for the Priest. With my method described above I felt very powerful and I never had to use seals at all.

TRUE. But, I started in the Cemetery/Crypt and the zombies are slow.
Better than starting with the mines and going mad with spiders and super-healthy trolls.

Like maggotheart said above - Must focus on one magic school !!!
I try to put 6 to Melee/Armor/Ranged - to be able to use better gear (as good as Priest class can use, anyway), then add only to magic.

Going ALONE, why? I don't think the XP splits. Hugo has great melee damage and just like Diabolus have over 300 of heath. I find them VERY helpful.

Priest is best job in the game

i am a knight now already over lv42 in act3, hp 573(almost game limit), max hit 198
and my friend is a thief

thief is good in lock-pick and steal
knight got nothing good at, only people respect you
but both of them have to learn some magic

the game has a balance system
if your defend and melee attack will be limit
also if you have good in reduce magic damage to you then your physical resistance will be very low
like a knight the magic skill can only up to lv10, only know student spell

i hope you won't get tired in this game
for my

Guys really, i just did as you suggested. Almost everything in inteligence and some speed. I choose inquisitoral magic to max for open wounds spell. Cleared bats around city, then all place of execution, then half cementary before skeletons, next all Asherbelt and now i can fight with ogres/trolls or go iron mines. But as i said i'm lv 10, 6lv divine magic for better bless spell, 10lv inquisitoral magic , 8lv wisdom and open wounds its only good for killing spiders, ogres/trolls take sooo long. So how have you cleared everything without seals (wtf)?
Word of an angel as main spell?

Try getting some magister items to pump up your magic skills, that should help with damage. Ogres and Trolls just have a lot of hit points, it took me a lot of castings to kill them at first. Single them out and work them over until they die. Use fireblade to give the companions some extra damage. My school of choice was Miracles and I was using Word; I don't know about Open Wounds, Im assuming the damage potential is comparable across different schools but I could be wrong. I found orcs a little easier to bring down than Ogres, maybe fight them until you level up a bit?

For seals, I used them as door openers and sold them for cash but never needed to use them against enemies.

I also built a priest on hard mode and did many things similar as the original poster but here's what I did differently.

First, I used the console to level up a priest to level 45 and maxed the magic skills at 20 each so I can test the spells to see what magic school to focus on. The descriptions of the damage for some of the schools appear to be wrong. The way damage works for many of the spells is it starts at 1d6 or 1d4 or something similar and then goes up based on skill points in the magic school so if you have 8 in a school it would be 8d6. Based on my tests of the spells (excluding the magister level spells since you don't get those until level 45), the best spells in order are:

1. Locust Swarm - it's a student level spell so you get it at level 15 and when you get it to magister level it does a ton of damage, can be recast every 1 second with maxed wisdom, and has a low mana cost of 11 with maxed wisdom. You don't need a direct line of sight to cast it either
2. Lightining - 20d20 at skill of 20
3. Open Wounds - 20d4 at skill of 20. The description for this one is wrong as it seems to do similar damage to Astral Attack and Stoning. The advantage is that it's a novice spell so you get it right away
4. Astral Attack - Similar damage to Open Wounds but it has a recast timer of 0 (with wisdom maxed at 20 and at magister skill level) instead of 1 second for Open Wounds but is a student level spell so you don't get it to level 15 vs level 1. Also, since you don't get the main benefit of it until a high level Open Wounds is better.
5. Stoning
6. Word of an Angel - lower damage than all the others but ok for the first 15 levels

Here's how I assigned my attributes:
10 Int, 10 Con, 5 Speed at character creation.

Then, focus on getting Int to 40, then speed to 40, then Con to 30. Con is needed for traps and things like that rather than damage from enemies. Also, spellcasters and fire bats can still damage you so you want some hit points. After getting those stats, you can focus on increasing Int.

Magic School Strategy:
Books in Divine, True Faith, Inquisitorial, and Miracles are available from the start of the game so feel free to choose some of the others when creating a character and then buying a book or two if you want the other magic schools.

The best school to focus on is Miracles which has four really useful novice and student level spells, Insight, Word of an Angel, Locust Swarm, and Levitation. Insight raises perception to help you find treasure and also increases you spell penetration so your spells hit more often. Word of an Angel will be your damaging spell until level 15 when you get Locust Swarm. Levitation is useful for avoiding damage which is especially useful in the beginning of the game when your hit points are low.

Skill focus:
Since your spells go up in damage significantly with skill, it's better to focus on one skill in the beginning. Focus on getting Miracles to 20 by level 15 and when you get Locust Swarm, it will do a lot of damage and will make you very strong at level 15. As you continue to gain mastery levels, the damage over time increases and the spell becomes even stronger. After maxing Miracles, max Wisdom to decrease mana casting costing and time to cast spells. I also put 4 points into perception so that I would be able to spot treasures easier but this may not be needed if you have Insight cast all the time.

After maxing out Miracles and Wisdom, focus on any other spell school that's interesting. I also recommend getting Inquisitor as a spell school and getting the Sixth Sense novice spell as it will identify all of the items in your inventory (no skill points needed for that though).

What to do at the beginning of the game:
Use seals until you get to about skill level 7 (level 3) or 9 (level 4) in Miracles. Then use both seals and Word of an Angel. Also, don't sell your seals even after you stop using them as I use them for breaking doors / chests.

I cleared out the Hillbrandt Cemetery first, then Asherbalt, then Road to Dragon Rock, then Dead Tree area. After that, I was able to go anywhere.

Potions:
I had about 16 of each type in my inventory until level 15. Then, when you get Locust Swarm, they will start piling up because you will be killing the enemies much faster than your companions use them.

Equipment:
Focus on getting + to all stat items which you can get when you become an Inquisitorial Prosecutor, from the Boots from the guy in the tavern and the Saint Ambrosius's skin armor. I only started focusing on this when I got the ring and not the first two items so this isn't super important but it will help if you get Con as it will provide a lot more hit points and allow you to focus on Int. At the beginning of the game, I bought items that gave + exp%.

For the magical boxes you find, save before you open them and select you need nothing and get a free attribute point. If you don't get one, reload and try again. You can also get free skill points instead if you want but I prefer attribute points.

I'm currently level 22 right now near the end of Act 1 and the game has been easy on hard. I have about 67,000 in gold as I never have to buy anything. This could be used on buying more magical boxes to increase stats.

bump for act2 update

Thanks guys - this is super helpful!