Razakel

I probably can be more competitive in Modern, for that just take out the Dark Rituals, you can add Crypt Ghast or Magus of the Coffers or Liliana of the Dark Realms to help you get mana and finish with (all swamps have: {T} 4 Black Mana Emblem)

I agree with some people here, Corrosive Mentor isn't doing much, giving Wither to creatures with Wither is pointless and you get room for 3 more cards :)
You can switch it for another Crumbling Ashes + Grim Affliction + Sudden Spoiling/Black Sun's Zenith. Giving Wither to creatures with Wither is pointless and you get room for 3 more cards :)

Tried to make it around an average price, not to high nor low that i would cheap out on some cards, but mutavault isn't really that cheap so if you're willing to spend some extra, mutavault is indeed a great idea to add to this deck :)

Already considered Brave the Elements, Mutavault it's just expensive and Honor of the Pure would be great but this deck shouldnt need more pumping up since it uses Doubtless One's to attack while other clerics exile or nullify the damage done by the opponent.
About the lands, if you pay attention, creatures are somewhat heavy and that extra mana should provide a faster output and speed up the game in order to avoid getting stuck with 2 or 3 lands and a very slow game...

You should consider Browbeat, Wheel of Fate and Wild Guess to refill your hand.
Also Guttersnipe for extra damage for every spell or instant you cast.

If you want to make a way to quickly bypass most defences i sugest Galerider Sliver. It enters fast and gives flying.