VI:
Castiel Rises to find that he has Fallen.
Heaven has forgotten him, he thinks at first, for he was dead, after all. Perhaps that is why he feels only the slightest bit of Grace within him, just enough to make him an Angel instead of a Man.
Almost as soon as he thinks that, he knows it is wrong. Whatever power brought him back was far too great to have been overlooked. They have simply left him behind, stripped of his former glory and grace.
Heaven has lost faith in him, and for the first time his own conviction begins to waver.
VII:
When he kills a Brother for the sake of a man, Castiel realizes that Heaven's actions were justified. If he had all of his Grace, he would surely use it to storm into Heaven and wage war.
Still, as his sword pierces Light and Wing, he feels something dangerously close to hate. How have they retreated so far from Father's Will? Mankind was created in Him image, and Castiel fights only for mankind.
(It occurs to him that perhaps it is Father's job to protect His other helpless sons, but that thought is thrust out as he stabs another Sibling.)
VIII:
Jimmy Novak never wore any jewelry besides his wedding ring, and maybe that's why the amulet that Castiel takes from Dean feels so unnatural. It hangs around his neck, heavy and glaringly obvious.
But, everything is odd about being in a vessel. He accepts the minor discomfort without any complaints.
Eventually, he comes to think that the amulet was hanging as burden that chained him to his quest. Leaving it would be freeing himself from the difficulties of upholding faith, and that would be weak of him.
He never used to need something to remind him of his faith, but times are changing.
IX:
When he takes Dean's amulet, Castiel promises that he will not let himself lose faith. Rather, he will expand it, finding faith in Dean, in Sam, in that they will stop this. That they will stand against their foes in the end, with Father on their side. That soon, the amulet will burn against his skin. And he does.
But, slowly, he loses hope. Hope is not faith, he tells himself; and it's only the hope that his brothers will stand with him. It will not hurt him to stop deluding himself.
He thinks that when he returns the amulet as well.
X:
Heaven is gone. Father is gone. He has no reason to keep his faith.
He does anyway.
It is faith not in that they will end it, but in that it will be over soon.
He does not pretend that they are the same things, but they keep him moving. Faith in the End moves him along, dumb and dazed and utterly alone, even with Sam and Dean next to him.
He condemns everything else. Let Heaven fall; let the Humans only keep their belief in Father.
When the end comes, he will not have lost anything more, and he desires that above all else.