The Age of Utopia: Christendom from the Renaissance to the Russian Revolution, by John Strickland

This really is a wonderful image.  Christian or not, "up" is the direction of progress, perfection, improvement, liberty.  "down" is the opposite.  Of course, for Christians, there is an additional idea that comes with "up" and "down": that would be heaven and hell.  One need not understand these to mean physical places to be explored by spaceships or deep-mining.  It is the imagery that is important, one that we all "understand."

Further: the descent.  How far down can one walk from the mountaintop while still retaining much that is good from the heights?  Clean air, fresh water, beautiful trees, etc.  How long before one notices that he is surrounded by foul air, stagnant water, and dried and dead shrubs?  In other words, even when falling from the heights one retains the memory of the heights for a time because of the very gradual transition.

As far as the Christian West goes, one can trace this to whatever bogeyman one chooses: 1914 and the start of the Great War, the Enlightenment, 1517 (and the corruption that made such an event inevitable), or – as Strickland posits, 1054 and the Great Schism.

My personal choice is the Enlightenment, and my reason is that it was at this time when man purposefully divorced himself from God.