METATRON:It had to be your grace, Castiel. The first trial was to kill a Nephilim, the result of the love between a human and an angel. The second trial was to fetch cupid's bow, an angel with the power to make two humans fall in love. So, you understand why it had to be your grace to sacrifice for the third trial.
CASTIEL:I'm not sure I do understand.
METATRON:The third trial, Castiel. 'An angel who can love a human above all else is to sacrifice his grace.'