The true picture of the past flits by.  The past can be seized only as an image which flashes up at the instant when it can be recognized and is never seen again.  "The truth will not run away from us": in the historical outlook of historicism these words  of Gottfried Keller mark the exact point where historical materialism cuts through historicism.  For every image of the past that is not recognized by the present as one of its own concerns threatens to disappear irretrievably.  (The good tidings which the historian of the past brings with throbbing heart may be lost in a void the very moment he opens his mouth.)
           The second sentence of the passage summarizes Walter Benjamin thought on history.   He is simply saying that the past can not be relived nor re-witnessed.  History only comes up as an image, whether it is in our head or through pictures.  Gottfried Keller, who believes in the ideology of historicism, says that our history is very important to us.  I understand it to mean that if people do not look at our historical past as incidences that concern us presently, then we might as well forget about it.  There is no point discussing history unless we are looking at it with seriousness.