The longer eternity is, the more inscrutable
Das Meer der Ewigkeit,
  The     ocean          of eternity,
     je mehrs der Geist beschifft,
         the  more it    the      mind      sets sail on   [beschiffen = travel by ship]
Je undurchschifflicher
  The more  impassable      [untranslatable play on Schiff = ship; lit.: the more un-through-ship-able]
     und weiter ers betrifft.
          and     wider   (the mind) it finds. [sense: the more one sails, the wider and more inscrutable this ocean becomes]