Everything that is hidden will be made visible...for a lamp's place is the lampstand and not under a bushel basket. Today's Gospel reading is a follow up of the Parable of the Sower that was the content of yesterday’s Gospel reading. If the parable of the sower left the apostles lost, I bet the teaching about the lamp light didn’t fare any better. What was this that had been hidden? Was there a lamp that had not been put to its proper use? 
Jesus began his ministry by proclaiming a time of fulfillment, that is, the in-breaking of the kingdom of God. The beginning of Jesus' ministry inaugurated the fulfillment of the promises which God had consistently made to God's people from the very beginning of creation as is made clear by the history of God’s journeying with the chosen people, Israel. It was in Jesus’ teaching and ministry that these promises were to be fully realized because Jesus is the full revelation of God.
The nation of Israel had been chosen by God as the conduit of God's revelation to creation. As the elected nation, Israel was to be a means of God’s blessing upon other nations (cf. Gen 12:2), as well as being a light to the nations (cf. Isaiah 42:6). Israel was to lead the nations into acknowledging the one true God through her godly actions (opening eyes of the blind and freeing captives from prison and dark dungeons [cf. Isaiah 42: 7]). This is where Jesus picked up from when he began his public ministry (cf. Luke 4:16ff). Jesus presented his life and ministry as the coming to fulfillment of God's promises. The revelation that had been limited to Israel up until this moment was now being made accessible to all. The knowledge of God that had been limited to Israel was now being made accessible to the nations. The knowledge to salvation that had been revealed only partially to the nations was now being made accessible to them. In Jesus, Israel was now taking her place in the lampstand as the light to the nations. The twelve, who had said yes to Jesus’ summons, were now part of this important mission. As partners of Jesus, they had taken the responsibility of Israel upon their shoulders. It was now their responsibility to lead the people towards the light of God’s salvation.