The teacher of Israel (ho didaskalos tou Israēl). The well-known or the authorized (the accepted) teacher of the Israel of God. Note both articles.
And understandest not these things? (kai tauta ou ginōskeis). After being told by Jesus and after so propitious a start. His Pharisaic theology had made him almost proof against spiritual apprehension. It was outside of his groove (rote, rut, rot, the three terrible r’s of mere traditionalism).