Diminishing Return

There are currently 3 stats that have a build in diminishing return these are armor, blocked amount and resistance. The reason they have a build in diminishing return is because otherwise the stats become better the more you have of it making you want more resulting in it become even better etc.

The formula for these 3 stats are as followed

Armor: 

Damage % reduction = Armor / ( Armor + 6500 ) * 100%

Block:

Blocked % = Block / ( Block + 721 ) * 100%

Resistance:

Damage % reduction = Resistance / ( Resistance + 600 ) * 100%

Magic damage

First of mitigation talents stack additive and resistance stacks multiplicative with that reduction so currently cleric tanks can get 28% magic damage mitigation (15% from Shield of Faith, 3% from Thick Skinned and 10% from Thorvin's Law) and as a result our magic tanking capability's are lower then that of other tanks.
For comparison a warrior tank can have up to 52% magic damage reduction (57% if the target has mana) and as a result takes 50% less magic damage.

For those wondering why having 52% vs 28% is 50% less damage taken
Lets look at a magic attack that would deal 10000 damage.
A warrior will take 10000 * ( 1 - 0.52 ) = 4800 damage.
A cleric will take 10000 * ( 1 - 0.28 ) = 7200 damage.
( 7200 - 4800 ) / 4800 * 100%= 50.0% more damage taken

Now I didn't take resistance from raid buffs into consideration so lets add that.
That's 30 from planar protection and 40 from bard/archon buff for 70 resistance to all schools.
70 resistance = 10.45% less damage taken.
So a warrior now takes 1-(1-0.52)*(1-0.1045)=57.02% less magic damage.
And a cleric now takes 1-(1-0.28)*(1-0.1045)=35.52% less magic damage.

That same attack now hits for the following
A warrior will take 10000 * ( 1 - 0.5702 ) = 4298 damage.
A cleric will take 10000 * ( 1 - 0.3552 ) = 6448 damage.
( 6448 - 4298 ) / 4298 * 100%= 50.0% more damage taken

So as you can see the raid resistance buffs have no effect on the effective difference between a warrior and a cleric. (and even if we fully take Favored of the Valnir into the calculation as full mitigation (this means no overhealing at all which is not the case) we still take 35% more damage)

The counter
To partly counter the effect that we take more magic damage it is possible to stack resistance of the type you are encountering. Currently that is mostly life is Greenscale's Blight and Death in River of Souls and with Hammerknell you can expect water damage.
There are a couple of places where you can pick up resistance.

1. Dark harbor from the cabalist tree provides 40 earth, water, air and fire resistance.
2. Bard/Archon buff provides 40/39 resistance to everything.
3. Planar protection guild perk that can be used to grant up to 30 resistance.
4. Resistance runes for chest and shoulders they come in a 21 and 27 resistance version for a total of 42-54 resistance.
5. The river of souls world-event gave the option to get a trinket with stats that included 20 death resistance.
6. The Hammerknell world-event gives you the option to obtain a trinket with 20 Water Resistance.
7. The Hammerknell world-event gives you the option to obtain a sigil of the five with 20 Water Resistance which is perfect for making a water resistance sigil.
8. Lesser essences up to 6 in total (if you make a resistance sigil/foci don't use greaters)
   Ordered from best to worst.
   1. Inscribed Sourcestone Resist essence provides 24 resistance + 17 endurance
   2. Crystal Sourcestone Resist essence provides 26 resistance
   3. Inscribed sourcestone essence provides 16 resistance + other stats
   4. Raid rift essence provides 15 resistance + other stats
   5. Crystal Sourcestone essences provides 12 resistance + other stats
   6. Expert rift essences provides 11 resistance + other stats
   7. Green essences with 1 other stat can provides up to 12 resistance (strength or endurance are the best)
   8. Green essences with more then 1 other stat can provides up to 6 resistance
   9. The collector's edition weapon rune provides 10 resistance of one kind

Then one last note about resistance, it does not scale linearly but like armor and blocked amount it got a build in diminishing return.

Stats for tanking

As a cleric tank there are a lot of stats that are usefull to you and here I will show you what each stat does assuming you have atleast picked up the basis to tanking spec (see below at tanking builds).

Toughness: Reduces bonus crit damage taken from non player sources. 1Toughness gives a 2% crit bonus reduction.
Endurance: Increases you health by 9 per point. We get a 90% endurance buff from MoL and as a result 1 endurance on your gear will give you 17.1 health.
Wisdom: Increases your spellpower by 0.75 per point and also some mana regeneration.
Intelligence: Increases your spellpower by 0.25 per point, your max mana by 10 per point and your spellcrit by 1 per point
Strength: Increases your Attack power by 0.5 per point, your block and parry by 1 per point.
Dexterity: Increases your Attack power by 0.5 per point and your dodge by 1per point.
Focus/Hit: Increase the chance to hit with spells/melee hits by 0.2% per point. (1 focus also increase hit by 1)
Attack Power: Increases the damage of your melee attack.
Spellpower: Increases the damage of your Spells, gives you 0.9parry per point, 0.6dodge per point and 0.3block per point.
Parry: Increases your chance to parry an attack by 0.01618% per point resulting in 0 damage taken.
Dodge: Increases your chance to dodge an attack by 0.02369% per point
Block: Increase your chance to block an attack by 0.04854% per point and also effects the amount you block for (for blocked amount see diminishing retuns).

The caps
Some stats have caps that effect how high the priority of each stats is.

T1 Experts:
Toughness: 50
Focus and Hit: 70

T2 Experts:
Toughness: 100
Focus and Hit: 120

T1 Raids:
Toughness: 150
Focus and Hit: 220

T2 Raids:
Toughness: 200
Focus and Hit: 320

General:
Parry: 20% (1236 parry rating)
Dodge: No cap at all.
Block: There is a soft cap at 45% block chance (from rating alone and doesn't affect block from PoR)
Armor: No known cap atm

The Priority
The stat priority assuming a raid environment (but even in a non raid buffed situation this is the stat order you should gear for)
Toughness (until cap) > Focus (until cap) > Endurance > Strength > block rating > Spellpower > Wisdom > Dexterity > Dodge > Parry > Intelligence

Toughness, focus and block are as a result the stats you should be looking for the most on gear.

For more information regarding stat priority and stat values I advice looking at this thread on the official forums: http://forums.riftgame.com/showthread.php?205793-Justicar-Speadsheet