Why Is Jerusalem Called The Holy City?

A fifteenth century map of the world which is housed in the Vatican shows Jerusalem as the center of the world, and from this place radiate lines to the various continents and major cities of the world.  When I first saw that map, I smiled, but as I grew in understanding of how significant Jerusalem is to our world, no longer do my lips crease in amusement.  The ancient cartographer who drew that map also knew something of the theological importance of Jerusalem.