In the fast foward.....He believes in the power of the island...

I'm not sure if that's what he believes in but he did seem to have some faith. He says forgive me when he's standing on the 6th street bridge, then tells Kate about praying (I forget the exact line). Maybe he and Locke exchange spiritual points of view? :shrug:

I get the feeling he comes to this conclusion without the assistance of Locke. I got the feeling when he went to the coffin (which I believe was Locke's) that he felt regret. It's like he can't apologize enough for his mistake. He knows he "wasn't supposed to do that."
If the year is 2010 then he's had a few years for this to all sink in and change his point of view from the doctor/leader, to the hopeless/drifter that he's become.

Jack doesn't believe in ANYTHING in the flashforward. He wants to go back to correct his mistakes.

There's a line from early in the series where Jack tells John that he doesn't believe in destiny and John replies, "You will." Now he does. "We weren't SUPPOSED to leave."

while I beleive that the person in the coffin is someone we have yet to meet...I do know Jack said we werent supposed to leave the island...and he did say the forgive me part when he was on the bridge...

Not sure if he switched places with Locke...but I do know he became a man of faith which is not what he was in real time..he was the man of science...which is why he is probably so screwed up right now...nothing he believed in was/is true...he never had control