Castiel knew exactly what was being implied by that. The larger picture. One without Ruby in it.
The Winchesters had taught the Angel a lot, more than perhaps he had previously realized. The value, the true value of love and loyalty. He had never questioned his devotion to his Lord, and still didn't. But he felt that he understood love more now than he ever had. 14 billion years in existence and there was still so much to learn. And it led him to one conclusion.
He loved Ruby.
He had a calm smile as he looked at the Almighty.
"As you wish."
With a bow he turned and walked out of the warehouse, his movements not hurried or scared. He knew what he was to do and he just prayed (oh the irony of that) that he would not be killed for his transgression.
The bigger picture. There were many angels who could take his place. But Ruby was unique. And he loved her.