What is happening in the Assassin's Creed Franchise?

I have beat the main storyline in AC 2 and AC Brotherhood and was very confused because I only got about halfway through the original AC game. I am looking for everything that has happened and what we will expect to learn about in the upcoming AC Revelations game. What conspiracies are going on in the series and how does the idea of religion fit into the storyline, i.e. what differs it from Christianity and others. A lot of information is needed, thanks!