Matthew 5:13
“You are the salt of the earth; but if the salt loses its flavor, how shall it be seasoned? It is then good for nothing but to be thrown out and trampled underfoot by men.
Salt serves several purposes. And when those purposes are lost because it loses its properties, then it is tossed out. In those days it would be thrown out into the path or road to help keep them from being overgrown.
Salt is most used to give flavor. Kitchens usually have a supply for cooking. Ultimately the purpose of salt in the kitchen is to cause a change. A change that improves the subject. Often making something that tastes blah into something that is delicious. As Christians, one of our calls is to bring Jesus into lives around us. Changing them into something wonderful!
It is also a preservative. Coating meat with salt will make it last much longer and in the days of no refrigerators, this worked very well. As we bring God into the lives around us, we bring Gods preservation into them in the process.
When we begin to lose our connection to God and seek our own way, focused on our success and gains, we begin to lose the values that salt has spiritually. People around us do more changing of us than we do of them. And God loses his preserving ability through us. Then, for spiritual purposes, we become useless.