Bless Skills Heal
Description: Primary cleric skill. Heals hp. Currently heals to max with single use, may change in future updates to heal based on luck stat.
Requirements: Level 5.
Required for: None known.

Notes: ALL clerics should learn Heal ASAP. Its usefulness cannot be overrated. Leveling increases MP cost, but may decrease cooldown time. Currently no need to level beyond 1, except for quickness.

Purify
Description: Heals status effects.
Requirements: Level 10.
Required for: None known.

Notes: Not useful yet as there are no status effects to be healed. To be avoided until further updates.

Avenge
Description: Raises critical atk damage and rate for a set amount of time. Effectiveness, time limit, and MP cost increase with skill level. ATK buff.
Requirements: Level 15.
Required for: None known.

Notes: Hasn't been useful for me, but most other clerics seem to like it. I get criticals half the time anyway.
Buff Notes: The effectiveness at level 1 can generally be multiplied to determine the effectiveness at each additional level. EX, 1% more critical hits at level 1, 2% at level 2, etc. Casting an ATK buff will cancel any ATK buffs already on the target char. The same is true of DEF buffs. From what I have seen, all buffs lst 40 seconds at level 1, and increase by 10 seconds per level to reach 130 seconds (2 min, 10 sec) at level 10

Sanctuary
Description: Raises DEF, Def lvl(evasion), and MDEF for a set amount of time. Effectiveness, time limit, and MP cost increase with skill level. DEF buff.
Requirements: Level 20.
Required for: None known.

Notes: Extremely effective at level 10. 3 points DEF, 4 points MDEF, and 40 points evasion per level, the end total at level ten is 30 DEF, 40 MDEF, and 400 EVD. A must for any cleric, whether in a party or solo.

Focus
Description: Raises accuracy for a set amount of time. Effectiveness, time limit, and MP cost increase with skill level. Effectiveness cannot be determined as it adds an "accuracy" stat, rather than damage level, which doesn't actualy exist on the char info page. ATK buff
Requirements: Cannot recall, possibly level 30.
Required for: None known.

Notes: Between the two ATK buffs, I would choose Focus. I cannot confirm the effectiveness at this time, but accuracy has always been more of a problem than critical rate for me.
Update: Upon close inspection and testing in DC4, I have found that focus really does seem to work. I miss about 50% of my attacks without it, but I only missed 1 time out of about 50, or 2% of my attacks, with focus active. It could be coincidental, but it seems unlikely that that is the case. Combined with the physical skills and sanctuary, focus can make any cleric almost as effective as a warrior in front line combat. If only there were a buff to raise your atk damage...

Fortitude
Description: Raises block rate of shields for a set amount of time. Effectiveness, time limit, and MP cost increase with skill level.
Requirements: Level 35, I think.
Required for: None known.

Notes: Useless skill for two reasons: 1. It isn't very effective for chars that don't use a shield, and only swrod shield warrios and Priest type clerics use them. 2. Cancels out Sanctuary, lowering total possible defense when active.