The character will be ambivalent towards the party, of good alignment but self-righteous in his certitude, and horribly horribly misguided. Left to his own devices, he'll eventually become a force of terrible destruction and a major antagonist (I use the more disinterested cleric/deity relations from the Eberron setting, so alignment changes will not shut down his cleric spellcasting). If he does have an alignment change, I'd swap out Eldritch Disciple powers to suit his new alignment.

However, I could see the players intervening and either turning him to a more enlightened way or at least persuading him not to create catastrophes.

His ultimate level is of course highly dependent on whether/when the players get it into their heads that they want to kill him, but if he doesn't get curb-stomped by the party, he could reach 20 or even low epic.