O King of the nations, the only joy of every human heart; O Keystone of the mighty arch of man, come save the creature you fashioned from the dust.
In stonemasonry, the block at the apex of an arch is called the "keystone". An arch cannot support itself without the keystone; it is the piece that holds the rest of the arch in place and allows it to bear weight. Truly, Christ is our keystone: He is the head, the Church His mystical body. He gives us life and hold us in existence--with God we are strong ("I can do all things through Christ Who strengthens me," Phil 4:13), without God we crumble back to the dust from whence we came.
Only through Christ, our Keystone, can all nations be united in peace: the "arch of man," the coming together of all humanity, cannot bear weight without Him.
O come, Desire of Nations, bind
In one the hearts of all mankind;
Bid every strife and quarrel cease,
And fill the world with heaven's peace. Rejoice, rejoice; Emmanuel shall come to thee, O Israel.