How about this. The centurion, who stabbed Jesus isn't immortal simply because he was cursed. Maybe he picked up a denarians coin shortly after the crucifiction and has long since succumed to the Denarians will.
The Denarian will betray the necromancer and steal the power because by doing so releases him from the coin, turns him into a god and makes him more powerful than even Lucifer since he's devoured Power from the One who cast him out of heaven in the first place.
Just think about a fallen angel set free in the mortal world. One with free will. Nasty!
If you want to keep the Centurion as being cursed, think about what he's feeling when the denarian removes his influence. Maybe god cursed him for this very reason - this very opportunity to redeem himself.
I don't know how the PC's fit in...but I thought this train of thought was cool....
Maybe in the final battle, after the PC's have killed lots of necromancers and henchman and monsters and still failed to stop the denarian who is now surrounded by an inpenetrable field of magic, they have to use SOCIAL COMBAT to convince the Centurian (who is standing beside the Denarian) to do the right thing and stab/kill/remove item of power from the denarian before the denarian fully energizes.
As far as Judas goes, I say he's just a guy who clues the PC's in on what's going on.
On another note: I'd still start the ritual at noon and finish it at 3pm. It just seems to me that that is the time the power and spirit will be most closely tied to the mortal world. To make it all necromantic and creepy, just make Good Friday co-incide with a full eclips.