There is a passage of Scripture where Jesus compares his disciples to salt. Salt was very valuable in the middle east in Jesus' day. It was used both as a condiment ans as a preservative. But Jesus says specifically in the pasage that if salt loses its taste\flavor, it is worthless. It only makes sense to toss it out.
As I understand chemistry, this is virtually impossible. Salt (NaCl) is a very stable compound. It cannot really lose its saltiness. But Jesus definitely said it would lose its taste.
Salt was "harvested" from a few sources. It was dug out of mines. It was processed from water containing salt. In both cases, it was possible to gather contaminents\impurites with the salt. So, it was possible to get something that looked like salt, but would not taste like salt. Also, because salt (NaCl) is very water soluable, and the impurities were less water soluable, we could lose the salt if it was exposed to water.
People could go through their lives and have the "saltiness" washed out of their lives, or the "non-saltiness" added to their lives to the point where it overwhelms the "saltiness."