Apparently, Crusader Kings II is still missing something, which seems unlikely considering the amount of content that each major expansion has added. The Old Gods took the cake* - Forseti loves a slice of Battenberg - adding a couple of centuries to play with, a host of religions and new raiding mechanics mechanics. That's not the end of the road though. It seems the one God and the diverse groups that claim to know Him best is a little jealous of the Old Gods and the Sons of Abraham expansion is hoping to make Him feel better by honouring "the big three in Medieval Europe: Christianity, Judaism and Islam". More details below.
Hundreds of new religious events are the core of it, spread across the three major religions and the entities that are created by their schisms (not dimensional schisms, not world-eating entities). Christianity has the following to look forward to:
Plunge into the powerful and profitable world of Papal politics, appointing your bishops, gaining influence with the College of Cardinals and reap the rewards of the Pope’s money and favours. Show your devotion with the Holy Orders; their clout will come in handy when you want to expel troublesome relatives to a monastery!
Islamic lords will have philosophical and political decisions to make, "choosing to follow the rationalist Mutazili or opposing Ashari", and Jewish rulers will be able to rebuild the Kingdom of Jerusalem. History branches yet again. Due November 18th.
* added a link because my housemate reckons 'take the cake' is only ever used in a negative sense, like the very British 'takes the biscuit'. I am reclaiming cake seizure as a positive action.