What would happen if Christ-followers were to permeate society as agents of redemption? Jesus told His followers they were to influence the kingdom of this world by leading it to the kingdom of Heaven.
Salt: A Hidden Influence
Up on the mount, Jesus said: "You are the salt of the earth." There are only a few situations where salt should be seen - french fries, pretzels, and a Virginia ham come to mind. Most of the time, salt is at its best when it is worked in. Worked in to food to add flavor. Worked in to food as a preservative (or stop decay). Worked in to the soil to add nutrients. So, maybe Jesus was saying the disciples were left here to stop the decay of the world, and/or help the world bring forth fruit to God.
Light: A Visible Influence
Jesus also said: "You are the light of the world." Lamps were best used in two ways. First, a lamp was useful as a guide. A tool to show the way. Second, a lamp was useful to reveal. A tool to show what is behind the darkness. So, maybe Jesus was saying the disciples were left here to point the way to the loving God; and maybe Jesus was saying the disciples were left here to reveal the destructiveness of sin and show a better way.
Silliness: Unsalty Salt & Hidden Lamps
Christ-followers who lose their savor are as silly as salt with no saltiness. Christ-followers who hide the Light are as silly as a hidden lamp. No, instead we have a purpose! We are to "let your light shine before men in such a way that they may see your good works, and glorify your Father who is in heaven." At times we are to be hidden, and at times we are to be visible, but at all times we are to make a difference - for good (and God's glory).