"Things will happen as they will," Aziraphale said with certainty. "We don't always understand in the moment, but our Father means for some things to pass in ways that we cannot comprehend. He has tested you beyond what you might feel are your limits, but I can't help but think that you have proven a more than worthy servant of Him." He had to believe that, because the alternative - that God was absent and there was no right choice - was somewhat unfathomable.
"And you did choose, Castiel," he said softly. "That's what matters. You made the right choice and stood firm, even knowing what it would mean for you and what you would sacrifice. Dean will not face anything alone. He and Sam will always have one another, and you to watch over them. You are not meant to die, Castiel...I won't allow it." He would fight until the end of all things if he could protect his brother. Because he understood him and he respected him for making the right choice, rather than the easy one. Heaven had become too complacent.
"Gabriel won't do a thing to you," he said with conviction. "I may have spent the last six millennia on Earth, but I still know how to use a flaming sword." He missed that sword, and really should look into finding it again.
At Castiel's question, Aziraphale smiled warmly. "I very much doubt she'll be able to understand what you're going through," he said, "and there's a room for you at my flat, if you like." He just left out that it was his room. He and Crowley had moved into a flat above his bookshop after the fire. "You're welcome there, and we would love to have you around."