Castiel is not certain whether it is the proximity or the sound of her words or even the words themselves, but something about her actions invokes an involuntary shiver within him. For all he knows, it could simply be a malfunction within his vessel, though he's never known it to do such a thing before and he's done nothing that would cause it to behave as such.
"I do," he replies, standing ramrod stiff, looking down at her and waiting for her next action.
Her smile is comforting - it means she's not disappointed in him or displeased. That had been his primary concern, but he isn't certain why, considering Anna has always been of the opinion that one is entitled to their own individual thoughts. However, her confession affects him in an unexpected way.
"I...am sorry about your faith. But should you ever need someone to rely on, I am always here for you. It is...likely not comforting, but considering the very nature of belief, I find it important to admit that I do believe in you and always have."