During World War I, some Turkish soldiers tried to steal a flock of sheep from a hillside from Jerusalem. The shepherd, who had been sleeping, awoke to find his flock being driven off. He couldn't recapture them by force, so he called out to his flock with his distinctive call. The sheep listened, and returned to their rightful owner. The soldiers couldn't stop the sheep from returning to their shepherd's voice.
People indicate whose sheep they are by whose voice they follow.
The voice of Jesus is heard through his word which has been revealed to us by inspired men. Parable - an allegory is a form of a parable, but differs from it a little. A parable illustrates that which can and often does happen. Whereas an allegory draws out a similitude between that actually described and that intended to be taught. The Lord, by means of this allegory, showed them that they did not perceive his teaching, not because they could not, but because they would not.