"For everyone will be salted with fire. Salt is good, but if the salt has lost its saltiness, how will you make it salty again? Have salt in yourselves, and be at peace with one another." - Mark 9:49-50 ESV
"No one after lighting a lamp puts it in a cellar or under a basket, but on a stand, so that those who enter may see the light. Your eye is the lamp of your body. When your eye is healthy, your whole body is full of light, but when it is bad, your body is full of darkness. Therefore be careful lest the light in you be darkness." - Luke 11:33-35 ESV
At the beginning of the Jesus' sermon, we have learned through the beatitudes what the characteristics of his followers should be. Now as we start to move further into Matthew 5, the verses following begin to give a picture of what they are to accomplish in the world in which they live. In other words, they have learned how they are to be--now they are going to learn what they are to do...
These two elements, Salt and Light are not two different metaphors for the same function. As John Stott states in his commentary on the Message of Sermon on the Mount:
"In fact their effects are complementary. The function of salt is largely negative: it prevents decay. The function of light is positive: it illuminates the darkness...For it is one thing to stop the spread of evil; it is another to promote the spread of truth, beauty and goodness."
The salt in those times was used as a preservative due to a lack of refrigeration. Things such as raw meat and fish would spoil rapidly (especially considering the climate) if they weren't salted. Seeing as how these things were already dead, the decaying process can be slowed through a preservative, but not stopped. So it is with the human race. Since the fall of man in the Garden of Eden, all of the created order has been degenerating; in particular--Man--who was made in the image of God, while he still retains that image, it is a broken and decaying one (Genesis 6:5-7, 11-12; Isaiah 53:6; John 3:18-19; Romans 1:18-32, 3:10-18; 1 Corinthians 7:31; 1 John 2:17). Therefore the Christian's first role in this world is to act as a slowing agent to the retrograde process that has been brought about by man's disobedience and hostility toward God. To quote Stott again:
"What does it mean to be Christian? To begin with, we Christian people should be more courageous, more outspoken in condemning evil. Condemnation is negative, to be sure, but the action of salt is negative. Sometimes standards slip and slide in a community for want of a clear Christian protest."
As we salt ourselves in Christ-likeness, it should have an overflowing effect. We preserve by being in our communities. As I said earlier, the salt of their day was mingled with other things. This is much like Christians in the world; we too are together with non-believers. This is an act of grace by God who sends us out into all of creation that the influence of the gospel may saturate the cultures of the world in order to hold back the destructive effects of evil. But when we fail to do our job, it leaves us and them worse for the wear. John Calvin states eloquently on the admonition of Christ to his disciples,
"There is nothing better than to receive this seasoning, by which alone our tastelessness is corrected. But, at the same time, let those whose business it is to salt it beware lest they encourage the world in their own folly, and still more, that they do not infect it with a depraved and vicious taste."
To say it another way, if we allow ourselves to become too saturated with the desires of this world, we lose our effectiveness. Christ leaves no wiggle room here. To be anything less than salt is to be worthless (Hebrews 6:1-8; Revelation 2:4-5). We become nothing more than something to be trampled upon.
"I do not ask that you take them out of the world, but that you keep them from the evil one. They are not of the world, just as I am not of the world." - John 17:15-16 ESV
Walk in wisdom toward outsiders, making the best use of the time. Let your speech always be gracious, seasoned with salt, so that you may know how you ought to answer each person. - Colossians 4:5-6 ESV
Beloved, I urge you as sojourners and exiles to abstain from the passions of the flesh, which wage war against your soul. Keep your conduct among the Gentiles honorable, so that when they speak against you as evildoers, they may see your good deeds and glorify God on the day of visitation. - 1 Peter 2:11-12 ESV