I just watched on of the terminals in the MCC. It showed the Heritic Leader talking to 343 Guilty Spark where 343 explains that the Halos are actually weapons.
It got me thinking, why do the Prophets believe the rings are some "Gateway to Paradise"? Are we ever told how they came about to believe this?
Ultimately, we don't know, but I think the indications are that all the Hierarchs actually believe in the Great Journey. To me, Truth's ultimate plan doesn't make much sense unless he actually does believe —otherwise he would be nuking himself when Tartarus activated the ring.
The only fly that makes you wonder why they didn't figure it out is that they were questioning 343 Guilty Spark—you'd think that he'd let slip what's going on, since he's nothing if not upfront if you give him a direct question. But I think that might be where the Prophets shot themselves in the foot—they never asked the questions they needed to because they were blinded by faith.
I've always wanted a sort of side story detailing Truth's thinking behind his actions. As many have noted, how Bungie presented him in Halo 3 felt like it robbed him of some of his subtlety, but there's also the fact that it seems odd to go ahead with his Brute uprising plans if you're so close to salvation.
As for the Heretic Leader---that terminal was already in CEA, or at least parts of it. They chopped off the first bit, and then spruced up the animation and shortened some lines compared to the original (which is a shame, because I liked the idea of why there are no Heretic Jackals being that 343 Guilty Spark wasted all of them
)
https://www.youtube.com/watch?v=v105JzJrBmg