The power a person receives is the awakening of his own imagination to its creative nature. The Holy Ghost coming upon him is that awareness stirring to life from within. To witness is not to argue a doctrine but to know from experience that the maker of one's world was oneself all along. Jerusalem, Judaea, Samaria and the uttermost parts are not lands abroad but circles of a person's own life, from the innermost state outward to the farthest reach of his affairs.
The unfolding moves from the center outward. First the inner conviction, then its spread through everything a person touches. A person becomes a witness when the truth stops being a belief and becomes something he has undergone. What he has awakened to within, he then sees confirmed in every part of the world around him.