My favorite Zen riddle is "who were you before you were born?" This question stumped a star student who thought he knew everything. He went and faced the wall of a cave for a number of years to find the answer. When he returned to give the answer to his teacher, he asked why the teacher had not just given him the answer. "You would have hated me if I did," the teacher answered.
Nothing is permanent. And everything is eternal. That to me is what the mystic John is saying.