Salt was very valuable during Jesus' day. It aided in the preservation of meat and enhanced the taste of food. But another less commonly known use of salt plays a key role in our understanding of what it means to be "salt" on our world.
During the first century, the people of Galilee used dome-shaped ovens made of hardened mud. Salt was mixed with dried animal droppings: a common fuel, because the chemical reaction made the animal droppings burn hotter and longer. Over time, however, the salt lost the qualities that made it effective. So, when it was no longer fit even for being mixed with manure, the "saltless" salt was thrown out.
As believers, God calls us to "mix" with sinful people and yet keep our distinctive Christian identity. God sent his disciples into an evil world to live out the good news. They were not to lose their faith by absorbing the values of the pagan world, nor were they to be isolated from unbelievers