ANSWER: The interesting thing about salt is that although it is essential to life and good health on planet earth, too much of it can be harmful. Blood, sweat and tears are this mineral in them. Its importance has lasted for several millennia.
Anciently, the mineral provided the financial support for pursuits like art. In Roman times, it was so precious it provided a means to pay soldiers. Salt received the nickname of 'white gold' sometime during the Middle Ages. Our English word salary comes from the word salarium, a name given to the mineral when it functioned as currency.
Jesus, during his famous sermon given on a mount, stated that believers were the 'salt of the earth' and warned that it was possible for them to lose their 'saltiness.' If believers lost their "flavor," they would be of no benefit to others. They would then only be worthy of being "thrown out and trampled upon by men" (Matthew 5:13). We begin to find our answer regarding the meaning of what Jesus stated by noting the context in which it was given.
14. You are the light of the world. A city that is set on a mountain cannot be hid. 15. Neither do they light a lamp and put it under a bushel basket, but on the lamp stand . . . you are to let your light shine before men, so that they may see your good works, and may glorify your Father . . . (Matthew 5:14 - 16, HBFV throughout)
Jesus tells those wishing to follow him that they must make a difference in the world. The difference made is "the flavor" or the good works believers are to do that are to shine like a light on a lampstand and offer a witness for Christ.
Consider that for many years salt was used as an agent to slow the decay of meat and other foods so that they could be edible for a longer period. Christians who obey God and do his will act as a preservative of the human race and the earth by slowing down the moral and spiritual decay of the world around them.
Making a difference, however, even includes even more than our actions. According to the apostle Paul, we must also use our words like salt to season or benefit those we talk to (Colossians 4:5 - 6).
So, what did Jesus mean when he stated that salt which loses its flavor is only fit to be trampled? At the time of Jesus, houses were often built (or had added to them) upper rooms where guests could stay for the night or which could be used for special occasions. These rooms usually had floors composed of wood which had a special mix of plaster laid over them. Ordinary plaster used for building structures like walls was too soft for floors since it was prone to peeling, cracking and generally falling apart. Adding salt to plaster made it hard enough to withstand the wear and tear experienced by a floor. It also, however, rendered the mineral impure and unable to fulfill its role as a seasoning agent. In such an irreversible state, it was then only good to be walked on.
Christians, like salt, need to live as pure a life as possible so that they can maximize their ability to "flavor" or bless the earth through their good works. When true believers mix into themselves things like false doctrines, or the vain worship of God through unbiblical holidays like Easter and Christmas, they lose their purity and their originally intended purpose. They then can become useless and vain (see Mark 7:7 - 9).