I haven't seen "The Passion of the Christ", nor do I plan to (I love the soundtrack of that movie, though). I'm not religious either.
I'm sure there are many religions and idealisms mocked in this episode, not to mention historical events. The theme was kind of nice: how different people see and react to one movie they've all seen. Some see violence, betrayal, and death. Some see a religious message.
In the case of Cartman, you just see Jews.
While Kyle is forced to reconsider his entire life and religion, Cartman is trying to mobilize a movement against Jews as a tribute to his "god", Mel Gibson.
In the meantime Stan and Kenny, who didn't like "The Passion of the Christ" at all, are on a mission to get their ticket money returned from Mel Gibson - who doesn't seem to have all his marbles in one place. (I am not a fan of Mel Gibson either, so any and all references to his past movies are totally lost to me). The man definitely has a fixation on torture, though; true or not (most likely not), that was one of the funniest parts in the whole episode.
The idea is good, but something just didn't click in this episode. There were funny moment but the plot wasn't overwhelmingly hilarious.