So what does it mean to be salt? Why did Jesus use Salt? To find the answers I began to seek the purposes of salt. I might say that while some of the things I found were not new to me there were quite a few purposes of salt that did surprise me.
First, salt functions as a preservative. It is an antiseptic which performs the negative function of preventing spoiling meat from becoming even more rotten and corrupt. In tropical countries where there is no refrigeration, people rub salt into meat to keep it from going bad. In the same way, when Christians come in close contact with the sinful people of the world, the world is kept from progressing further into unspeakable foulness. Christians benefit secular society. Without their presence, the world would degenerate at an even faster rate. As a moral antiseptic, Christians keep the corruption of society at bay by opposing moral decay by their lives and their words.
Now we must realize that, like salt, Christians might irritate the wound at first. Remember, Christians are not sugar; they are salt. But I say, welcome the irritation that Christians bring about. Welcome the discomfort that the salt of the earth initially creates, because salt will greatly benefit you.
Second, salt is necessary for flavor. Without Christians and the gospel they proclaim and practice, this world would be more insipid. There is no true flavor found in the world. There is no true joy found in this world. Only the company of Christians gives flavor and joy to the world. We can prove this by examining the worldly parties and good times. What are they like? Boring, tasteless, and insipid. As Christians, we have the function to provide flavor and taste to the world. The gospel of Jesus Christ is the power of God unto salvation to everyone who believes. The world is without hope, and only the Christian, by his proclamation and practice, will point the world to hope in Jesus Christ.
Third, salt is white, symbolizing purity. Christians are to be in the world and yet live pure, different lives. As I said before, Christians should glory in their difference. They are citizens of heaven who are living in this rotting world for its benefit. As such, they must constantly resist the temptation of being like the world and must be like the Lord Jesus Christ. They must oppose the rotting and foul values of the world, and proclaim and practice biblical lifestyles. As we read in James 4:4, "Friendship [or conformity] with the world is hatred toward God." As the salt of the earth, we must be holy, pure and different from the world. In Romans 12:2 Paul says, "Do not conform any longer to the pattern of this world" and in James 1:27 we read that we must keep ourselves "from being polluted by the world." In 1 Peter 4:4 we read that the world thinks it strange "that you do not plunge with them into the same flood of dissipation." Salt is pure.
Fourth, salt also was used as a fertilizer. Christians must function in a way that promotes the growth of that which is good in the world. They must oppose evil and promote good. In God's common grace, there is much good in his world, and Christians must support and promote that which is useful to mankind. For example, Christians must engage in political life to promote policies that are helpful to families. Historically, Christians have championed many causes that promoted the good of humanity. They have suppressed evil and caused good to grow and flourish.
Fifth, salt causes people to become thirsty. In the same way, Christians should cause people to thirst for Jesus Christ and his gospel. If we live as Christians daily for God's glory alone in the midst of the world, the world will observe us and desire to know about Jesus Christ. Now, I don't mean all the people of the world will do this, but God's elect will. Through us God will generate within them a thirst for the living God, and they will come to us and ask, "Why are you different? Why are you so full of hope and peace and joy even in the midst of troubles?" When they do this, you then can tell them about Jesus Christ. But the important thing to remember here is you'll need to "rub" shoulder to shoulder with the world in order for them to see that you are actually different and not just a really nice person.
(from notes taken from a sermon-don't remember who.)