If I have to say something that itches my curiosity about Jack, that is for sure the tremendous change between the Man of Science Jack, and the Man of Faith Jack, since his last moments off the island...this is higly noticeable in several episodes (through the looking,for instance) ... so when I Saw in "because you left" the scene where Ben asks Jack "what did he say to make you such a believer?" I was on edge wating for the answer..
Obviously ,Jack's answer surpised me ,even dissapointed me, it was to simple to make Jack a believer", the most you could get from Jack with that reason would be a certain feeling of guilt,which isnot enough to change someone's beliefs like that... so I thought that Jack maybe didn't tell Ben the whole story, and I was impatiently waiting to hear what did John tell Jack "make him such a believer"
So when fimally in "life and death of Jeremy Bentham " that conversation appeared,I got a STRONG feeling that what John told Jack wasn0t convincing at all 1) about the "Left-behind-losties" going to die, I felt that it wasn't a little bit convincing for Jack 2) abut the Father-saying-hello thing, even when it seemed to have touched Jack's soul, he didn't seem to believe it, it was too crazy to make The Man of Science believe (because for me it was Still M.of S. at that moment) 3) and because of the way the conversation ended, t didn't seem probable that Jack would turn into a M of F at that point, EVEN IF HE THOUGHT ABOUT WHAT JOHN SAID LATER...
so the biG questiOn is ...WHEN DID HE BECOME THE "NEW JACK"???
My theory is that after he spoke with Locke, his father appeared to him, asking why he didn't believe in John in scene very much alike the Scene of the Bible where Peter falls in the water and Jesus asks him "Man of poor Faith, Why have you doubted?" Jack's reaction would be esceptical and I imagine him saying angrily "YOU ARE DEAD!YOU ARE DEAD" and his father telling him taht he is, but that what realy matters is that HE HAS TO COME BACK AND BRING THEM ALL ALONG WITH HIM, AND EVEN MORE, CHRISTIAN TELLS HIM WHAT HIS PURPOSE WILL BE ON THE ISLAND, SO THAT'S WHY HE TELLS KATE THAT"MAYBE THE ISLAND WANTS TO FIX THINGS BY ITSELF"AND THAT "HE WAS JUST GETTING IN THE WAY"
I know that maybe I'm just over analyzing it, but I think this would be more "buy-able" and more exiting than just trying to accept that he just turned into the "NEW Jack" because of the reasons Locke gave him