You are the salt and light of the world. Having presented the beatitudes as the summary of the good news of the kingdom, Jesus continues with his proclamation of the kingdom by reminding those who had agreed to join him in mission what their call was really about. He wanted them to be aware of the great responsibility that they had assumed by choosing to be his collaborators in ministry. Being a follower of Jesus is a great responsibility for it is 'other-oriented.' Using the analogy of the effects of salt and light, Jesus reminds his followers that theirs is a call to action, an action that is other-oriented. Salt and light have something interesting in common: the outcome they effect come from outside of them! It is true that alt brings out the flavor in food, but the flavor does not reside in the salt! Likewise, whereas one can rightly contend that a lighting device will have light in itself, only an individual with the potential to see can benefit from the light. In other words, salt and light in themselves are quite useless if there is nothing to receive them. They act as catalysts and as such need something to act upon for their usefulness to be felt.
As much as Jesus' statement might elicit from his disciples a sense of superiority complex, I believe it should do the exact opposite. For what Jesus is telling them is that, yes, they have work to do, but they are not starting from a blank slate. Something/someone has already happened out there! Yours is to go out there and help them make sense of what it is they are doing. They already have a life! All you have to do is bring flavor to it. That which the disciples have, the responsibility that they have been given, is not for their own benefit. It is not for their glory. It is for the benefit of those whose lives they flavor, those whose paths they light. All that the disciples end up accomplishing is for the glory of God. If people's lives are changed because of a disciple's actions, then he/she shall have carried out his/her responsibility of placing lights on a lampstand.