The phrase appears in Jesus’ famous Sermon on the Mount (Matt. 5–7). Matthew 5:13 states, “You are the salt of the earth.” Jesus was telling His disciplines that they were to serve as both a seasoning and a preservative in the world. If they were not doing this, they were useless.
Salt was a valuable commodity in the ancient world. Some people, including Roman soldiers, were paid their wages in salt instead of money. Our word salary comes from the Latin world salarium, meaning “salt money.”
When Jesus referred to His followers as “salt,” He wasn’t referring to a cheap everyday item, but to something valuable and important.
See 393 (Sermon on the Mount).