The light shines in the darkness, and the darkness has not overcome it. - John 1:5 ESV
"But the fallen human beings need more than barricades to stop them becoming as bad as they could be. They need regeneration, new life through the gospel. Hence our second vocation to be 'the light of the world'. For the truth of the gospel is the light, contained indeed in fragile earthenware lamps, yet shining through our very eartnenness with the more conspicuous brightness. We are called both to spread the gospel and to frame our manner of life in a way that is worthy of the gospel." - John R.W. Stott
As for our second role as Christians, being the light of the world, Jesus gives a simpler explanation. That is, at least it is simpler in our ability to comprehend what we are to do. While there can be some linguistic and historical wrangling as to what definition of salt we are to be in a metaphorical sense. It also gives us a better understanding of how these two functions of being salt and light compliment one another. Without salt to "prepare the way", in a manner of speaking, the light will have little or no effect.
"This Little Light O' Mine..."
As the light of the world, Christ describes his followers in two ways--the first of which is 'a city on a hill'. We are not to be hidden; we are to stand out. It should be almost impossible for us to stay hidden. We should always seek to make ourselves as visible as possible, to be in plain sight of the watching world. In a second metaphor, Christ explains this as a lamp that is used to light a house. He points out how ridiculous it would be for anyone to light a lamp and then try to conceal the light that it gives off. John Calvin comments on this issue:
"This means that they ought to live in such a manner, as if the eyes of all were upon them. And certainly, the more eminent a person is, the more injury he does by a bad example, if he acts improperly. Christ, therefore, informs the apostles, that they must be more careful to live a devout and holy life, than unknown persons of the common rank, because the eyes of all are directed to them, as lighted candles; and that they must not be endured, if their own devotion, and uprightness of conduct, do not correspond to the doctrine of which they are ministers."
The Christian's life is one of ultimate exposure and exhibition. We are to be on display for all the world to see. How else are we suppose to light the way for a world that remains in darkness? People everywhere are searching for a "better way". Take a trip on the Internet highway or a click of the remote control and we are immediately overwhelmed with all sorts of products of "salvation". Everyone has an angle promising to save us from the problems and misery of life. It comes in all shapes and forms--from money to miracle creams. Yet these things never truly satisfy and leave us wanting. We, by our fallen nature are conditioned for the darkness. As Jesus told Nicodemus, "man loves darkness more than the light".
"...I'm Gonna Let it Shine!"
Therefore it is the duty of the Christian to spread the light of the gospel into the world. We are to illuminate the darkened eyes by our example. This is why Jesus admonishes his disciples to take care that they always have the light shining. That we do not try to hide ourselves because of our own human frailty. Also, we must not allow ourselves to be extinguished by discouragement of our failures at times. Christ sustains us in both our failures and our successes. There are many examples in the Bible to point out the shortcomings of many mighty men and women, both old and new testaments, who had moments of faltering. But the thing that will sustain us through it all is that we remember that we are not doing any of this for ourselves. Our light is not to be shown for our own gratification and glory, but to the Glory of God Alone. We are in the business of shining the light that points to salvation in Christ as the only way, not through us. As Paul states to Timothy,
though formerly I was a blasphemer, persecutor, and insolent opponent. But I received mercy because I had acted ignorantly in unbelief, and the grace of our Lord overflowed for me with the faith and love that are in Christ Jesus. The saying is trustworthy and deserving of full acceptance, that Christ Jesus came into the world to save sinners, of whom I am the foremost. But I received mercy for this reason, that in me, as the foremost, Jesus Christ might display his perfect patience as an example to those who were to believe in him for eternal life. To the King of ages, immortal, invisible, the only God, be honor and glory forever and ever. Amen - 1 Timothy 1:13-17 ESV
So as we go about our business of being in the world and not of it, we must always remember that salt and light must go together like law and gospel--you can't have one without the other. Light the way for the watching world.
Hoobastank - Crawling in the Dark
Again Jesus spoke to them, saying, “I am the light of the world. Whoever follows me will not walk in darkness, but will have the light of life.” - John 8:12 ESV
"As long as I am in the world, I am the light of the world." - John 9:5 ESV
Therefore, having this ministry by the mercy of God, we do not lose heart. But we have renounced disgraceful, underhanded ways. We refuse to practice cunning or to tamper with God's word, but by the open statement of the truth we would commend ourselves to everyone's conscience in the sight of God. And even if our gospel is veiled, it is veiled only to those who are perishing. In their case the god of this world has blinded the minds of the unbelievers, to keep them from seeing the light of the gospel of the glory of Christ, who is the image of God. For what we proclaim is not ourselves, but Jesus Christ as Lord, with ourselves as your servants for Jesus' sake. For God, who said, “Let light shine out of darkness,” has shone in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ. - 2 Corinthians 4:1-6 ESV
that you may be blameless and innocent, children of God without blemish in the midst of a crooked and twisted generation, among whom you shine as lights in the world - Philippians 2:15 ESV
Soli Deo Gloria,