Rabbi Yeshua called His disciples the salt of the earth. Just like salt adds the flavor to food, His disciples add the flavor to the earth. A little bit of salt can make an otherwise bitter taste palatable. His disciples are to be the force for good and righteousness that balances the bitterness and ugliness of the world and all that is evil and wrong and wicked.
You are the salt of the earth, but if salt has lost its taste, how shall its saltiness be restored? It is no longer good for anything except to be thrown out and trampled under people's feet. (Matthew 5:13)
The ancient world used salt as a preservative. In the Temple, the priests salted the sacrifices to prevent putrefaction of the meat, and the Torah refers to an eternal, enduring covenant as a “Covenant of Salt.”
Just as salt flavors and preserves, Yeshua’s disciples are the force that preserves and repairs the world. Judaism expresses the concept of preserving the world as tikkun olam (תקון עולם), i.e., “fixing the world.” As disciples of Yeshua, we must be busy fixing the world, preserving and repairing a broken and hurting world.
The Master warned His disciples not to lose their saltiness. “Salt is good; but if the salt becomes unsalty, with what will you make it salty again?” (Mark 9:50). He said, “If even salt has become tasteless, with what will it be seasoned? It is useless either for the soil or for the manure pile; it is thrown out. He who has ears to hear, let him hear” (Luke 14:34-35).
Can salt lose its salinity? The Talmud records a debate between Rabbi Yehoshua and a school of Greek philosophers from Athens. In the course of the debate, the philosophers attempt to stump Rabbi Yehoshua with a series of riddles. At one point they ask him, “When salt becomes unsavory, how can it be made salty again?” Rabbi Yehoshua answers the riddle with another riddle. He says, “Salt may be made salty again with the afterbirth of a mule.” The philosophers object, “Does a mule have an afterbirth?” Mules are born sterile, and therefore, they do not reproduce or have afterbirth. Rabbi Yehoshua explains, “And can salt lose its saltiness?” Just as a mule cannot have afterbirth, the laws of chemistry make it impossible for salt to lose its saltiness.
Yeshua played on the absurdity of the idea that salt could lose its flavor. The point is that it would be unnatural for salt to lose its saltiness. If other foodstuffs taste bitter, they can be salted into palatability, but this would not work for the salt itself. Just as salt without salinity would no longer serve any purpose, so too disciples who fail to live according to His teachings and the commandments of the Torah are no longer serving their purpose.