Dietrich Bonhoeffer's "letters And Papers From Prison

What makes a written work eternal—its message still so fundamental to the way we live that it continues to speak to us, hundreds or thousands of years distant from the lifetime of its author? Why do we still respond to an ancient Greek playwright's tale of the Titan so committed to humanity's survival that he is willing to endure eternal torture in his defiance of the gods? To the cold advice of a 16th-century Florentine exiled from the corridors of power? To the words of a World War I German veteran writing of the horrors of endless trench warfare?