O King of the nations, desire of every human heart; O Keystone of the mighty arch of man, come save the creature you fashioned from the dust.
King and Keystone, two images of the key piece without which everything below them falls apart. The king holds together his kingdom as surely as a keystone holds up an arch. Without Him, we are jumbled, fallen pieces; with Him, parts of a cohesive, functioning whole.
In the context of King and Keystone, today's antiphon's description of Christ as the "desire of every human heart" becomes self-evident: everyone desires wholeness, even if they don't realize that the wholeness they seek is only found in connection with Christ.
O come, desire of nations, bind
In one the hearts of all mankind.
Bid Thou our sad divisions cease
And fill the world with heaven's peace.
Rejoice, rejoice! Emmanuel shall come to thee, O Israel.