"But you will not." Castiel did not know where this confidence had come from; by rights he should have been cowed, humble. Zachariah was right - what right did he have to question his mentor's loyalty? To doubt their Father's will? And yet doubt he did, and questions he had, and maybe Ruby was right and being around humans had changed him for the better. Maybe Anna had spoken wisdom rather than blasphemy when she'd called them cold rather than perfect. "You need me to continue to influence Dean Winchester."
It was a bold move - some might have termed it reckless - gambling that Zachariah's own confidence was not so strong as to assume Castiel could be replaced, trusting that he'd done enough to prove that no other angel would be able to slot neatly in beside the Winchesters and earn even the moderate degree of trust Castiel had achieved during his tenure.
He refused to flinch even as Zachariah stepped forward - his mentor could have smote him with a thought, no doubt, but be that as it may, he would not give him the satisfaction of thinking him afraid - setting his jaw resolutely. "When this is over, Zachariah, you may deal with me as He sees fit. For the moment, this is precisely why Dean trusts me. Because I am asking you the same questions he asks me. Why did God wish you to make them forget, to give them those new lives?"