In this short comparison, Jesus points up to his disciples several truths. First, the great qualification of the teacher hinges, not on knowledge as such, but on discipleship. Only the converted need apply. The Jewish scribe was limited to old things, the law and its application. The disciple can draw on the newness of Jesus, because he lives for the kingdom.
Understanding is necessary for discipleship, and discipleship is necessary to further comprehension.
Jesus opens our sights to the faith as a teaching religion. The picture of the scribe widens to include all disciples. Israel put a lock on its treasure chest, wanting to keep God's blessings to itself. He draws out of his well of spiritual resources the divine riches. The action reminds us that treasures are for sharing, blessings are for blessing.