Holy Priests are dedicated healers - support.
You have good heals, good buffs, and your damage spells are based on your healing ability. Your damage output is not bad against a lonely target, and with the appropriate gear, you can be quite effective. But, what is the point in building a cleric if you want to focus on damage dealing ? So, this build is focused on helping others. Your damage spells will only be useful while soloing or when your tank is damn to tough for the enemies.
It's definitely a PvE build. For PvP, I don't have much experience about what you can do to help your mates. I don't feel Holy tree to be very effective in these situations, I would certainly go for a Blood Build or a real Hybrid Build if I planed to focus on PvP.
This build is technicaly not a Pure Holy Build, as I take one curse in it. But this curse adds to the support part of the build, and helps a lot for healing, as it debuffs enemy physical and magical attack.
All in intelligence. Intelligence up your healing, your magic attack, your magic defense, and your mana. You don't need anything else.
If you have to choose, prefer endurance over agility over strength. But honestly, most of the time, its useless to get bonuses in your other attributes.
Light Heal (5) : I don't see any drawback in Light Heal. It is, from the beginning of the game till the end the most efficient healing spell. The only other mono target heal being Grace, that you get very far away in the game, which had a tremendous cooldown, and which is not much better than heal in term of hp/seconds.
Holy Light (1) : Cool spell, but Improved Holy Light is much better. In the beginning of the game, you'll have much better spells to up, and in the end of the game, Holy Light is to light to help.
Saint's Strength (5) : Even if Saint's Strength is very powerfull, I won't put points in it before very late in the game. You need Angel's Blessing to maintain it. So, first, up Angel's Blessing, then, max Saint's Strength.
Healing Wave (1) : Very good spell, but cooldown is very high. Put points where you need them, and max Healing Wave only if you have to many of them (which I don't think).
Improved Holy Light (1-5) : I will max it late, that's why I say 1-5 for it's level.
Grace (1) : Put one point to get Circle of Healing. For me, Grace is to expensive.
Circle of Healing (5) : Your best mass heal.
Light Beam (1) : Very good spell, but you'll have Improved Light Beam at level 40, so, no need to up it more than one point. Don't forget to put it on your main attack as soon as you get it.
Spirit Infusion (1) : Good spell at the time you get it, but useless late in the game.
Improved Light Beam (1) : You get Master Light Beam at level 80, so, a bit far away. If you want, you can up it to make it your main damage spell. Personnaly, I won't put more than one point on it, because even if it helps, it's not your main role to deal damage.
Improved Spirit Infusion (1) : Good to have.
Master Light Beam (1-5) : Put one point on this one. If you realy want to deal damage, max it up. At the time you get it, you'll have some skill points available.
Guardian's Protection (5) : The more protected your tank, the easier your job. Plus, it helps you a lot while leveling.
Pure in heart (1) : Very good spell, works as intended. I'm not sure it helps to up it, as it only reduces CD. I would let only one point here.
Resurrection (1) : Very usefull for crystal tournament. One point is enough for normal situations, but many priests (whether pure holy or hybrid) like to have level 5 in this skill since it has only a 2 second cast time but restores 40% hp and mp, making it VERY useful in the Crystal Ladder
Angel's Blessing (1-5) : Hp regen is cool, but, honestly, you're not supposed to take too many blows. Mana regen is a must. You need it to use Saint's Strength. For me, any level in Angel's Blessing is cool, as long as you can cast spells with Saint's Strength up. If you plan to do lots of PvP, you may need to max it.
Circle of Power (0-1) : Prerequisite for Goddess' Embrace.
Goddess Embrace (0-5) : Very expensive in term of skill points, I would go for Goddess' Embrace very late in the game. It's very powerfull, but you have to know what you do.
(Scion+ only) Heavenly Protection (as high as possible) : This buff is definitely the most sought after buff in the game, but has a slightly different system for leveling the skill. Each level of the skill requires another 15-20 levels higher than the last level. The highest currently available level for it is level 6 (7 and 8 should become available once the level cap is raised to 120). Basically, if you get to scion, save points for this.
(NOTE: This part is under discussion and needs justification. I let it here, but am clearly not ok with it :
However, once you get to a higher level, curses are basically useless unless you have level 5 of all, which is a huge waste of 20 skill points. Most priests tend to have level 1 of each, which is a good balance. After reaching maybe around level 50ish, they become more for rooting people in PvP than for anything else)
Although this is technically a page for pure holy priests, curse of weakness is clearly a must in it.
Curse Mastery (5) : It's only a prerequisite.
Curse of Weakness (5) : Very good curse. It reduces enemy attack by 25%. But, enemy damage is reduced by far more. If the boss shoots your main tank, this curse will nearly reduce by half it's damage output. Plus, it does damage (in the beginning of the game, it's a good damaging spell), and it freezes (allowing you to use kiting on some monsters but it's hard).
Blood Beam skills and Tenacity skills are useless in this build. (Also, the Tenacity tree cannot be learned concurrently with the Protection tree)
Because you have high intelligence, you have high magic defense. Choose magic attacking monsters as your main leveling enemies.
I'll up skills that way.
2 : Light Beam to 1.
4 : Light Heal to 1. Always useful to have a healing spell.
10 - 12 : Curse Mastery to 4. Prerequisite
20 : Spirit Infusion to 1.
20 - 26 : Curse Mastery to 5 and Curse of Weakness to 5. It'll help you level up. And it's damn good. It's a personnal choice, but at these levels, you don't go often in dungeons, and you quickly reach level 35 anyway. So, maxing your solo ability seems more interesting than maxing your dungeon crawling ability.
28 - 36 : Guardian's Protection to 5. Now, you are a buffing/debuffing machine. I prefer that way to up, than maxing Light Heal first, essentially because Light Heal costs so much mana if you up it. The only drawback in going this route is if you find yourself with another healer in your party, with exactly the same skills. But I haven't found many. Most of the time, they up heal, and they'll be happy not to have to heal to much because of your buffs.
38 : Pure In Heart to 1. Prerequisite and usefull spell against Vega.
40 : Resurrection to 1. Now, you can resurect in Crystal Tournament. And when you see what Crystal Tournament gives you every day, of course, you want to resurrect your dead teammates.
42 : Improved Light Beam to 1. For soloing ability.
44 : Holy Light to 1. Always useful.
46 : Saint's Strength to 1. Prerequisite.
48 : Healing Wave to 1. Very useful.
50 : Improved Holy Light. 2500-3000 health during 12 seconds. For me, it's far better than having Holy Light to 5. And once again, mana efficient.
52 - 58 : Holy Light to 4 and Angel's Blessing to 1. Yeah, I know, it's very late. You need so much mana to use it, I prefer to put one level in Angel's Blessing, too.
60 : Improved Spirit Infusion to 1. It's a cool spell.
62 - 64 : Holy Light to 5 and Angel's Blessing to 2. At that stage, you must find yourself able to use Saint's Strength, heal, buff, debuff, and still be maxed out in mana all the time.
Now, you have everything you need. You can continue to max Saint's Strength, upping Angel's Blessing when needed. You can also begin to put points in your damaging abilities. Anyway, you are a holy priest, and you do your job in term of healing/buffing.