The disciples ask Jesus who is considered greatest in the Kingdom of Heaven. The question resembles scholastic disputes about how many angels can fit on the point of a needle, because it is entirely theoretical. Without understanding what the Kingdom of God is, they argue about how its hierarchy is arranged. In the same way, they divide up the places at the Saviour's right and left hand in the life of the age to come, without realising that they are asking for martyrs' crowns.
Jesus' answer is striking in its childlike simplicity and utter practicality. He does not discuss with them what they cannot understand, but offers something familiar and close at hand as an example. Christ does not speak of angels and archangels; He takes a child and shows in him the image of a citizen of His Kingdom. He does not encourage such theoretical discussions, but states directly what one must be like to enter it.
