VideoGamer visited a Civilization V: Gods & Kings 'behind-closed-doors' presentation during GDC 2012. They posted an article
that sums up a couple of details about the new Civ5 expansion pack.
• There are 20 different Pantheon beliefs to choose from, such as the God of War or the Goddess of the Hunt.
• Your pantheon choice rolls into your religion, and you'll also get to choose between two more bonus abilities. The first is the Founder belief, which gives you rewards for spreading the religion to other cities, and the second is the Follower belief, which will confer bonuses to any city (and any civilisation) that follows your religion.
• Certain units, like Pictish warriors for the Celts, generate faith when killing other units.