I like this alot. Really I like anything that gives the more linear classes some choice as they level up.

Is a Paladin obligated to take a mantle of faith every three levels or can he decide not to?

Also, just so I'm clear, when a paladin takes a new mantle, he immediately gets all of the powers for that mantle up to his level? So a paladin who takes abstinence at 19th level, say, would be immune to magical disease/poison and have the +2 bonus to mind-effecting spells?