And hast made us unto our God kings and priests: and we shall reign on the earth.  -Revelation 5:10

When did Christ make His people kings and priests? When could it be said, “And hath made us kings and priests unto our God?”…He made us kings and priests, virtually, when He signed the covenant of grace. Far, far back in eternity, the Magna Charta of the saints was written by the hand of God, and it needed one signature to make it valid. There was a stipulation in that covenant that the Mediator should become incarnate should live a suffering life, and at last endure a death of ignominy; and it needed but one signature, the signature of the Son of God, to make that covenant valid, eternal, and “ordered in all things and sure.” Methinks I see Him now, as my imagination pictures the lofty Son of God grasping the pen. See how His fingers write the name; and there it stands in everlasting letters-” THE SON!” O sacred ratification of the treaty; it is stamped and sealed with the great seal of our Father in heaven. O glorious covenant, then for ever made secure! At the moment of the signature of this wondrous document, the spirits before the throne-I mean the angels-might have taken up the song, and said of the whole body of the elect, “And hast made you kings and priests unto your God;” and could all the chosen company have started into existence, they could have clapped their hands and sung, “Here we are by that very signature constituted kings and priests unto our God.” ~ C.H. Spurgeon

http://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0010.cfm