Castiel is not required to tell Dean this. No one had said one way or the other if he should be informed. He had made the determination on his own that it is the right thing to do.

(It's what Meg would do. Explain things.)

He hopes he's calculated correctly.

"It was a test. To see how you would perform under battlefield conditions, as it were."

Under fire and for high stakes.

Castiel contemplates his hands, folded on his lap for a moment.

"And I believe it was a test for us as well. To see how effectively we would follow you."

"But that is conjecture on my part."