https://enduringword.com/bible-commentary/matthew-5/

a. You are the salt of the earth: Disciples are like salt because they are precious. In Jesus’ day, salt was a valued commodity. Roman soldiers were sometimes paid with salt, giving rise to the phrase “worth his salt.”

b. You are the salt of the earth: Disciples are like salt because they have a preserving influence. Salt was used to preserve meats and to slow decay. Christians should have a preserving influence on their culture.

d. If the salt loses its flavor…it is then good for nothing: Salt must keep its “saltiness” to be of any value. When it is no good as salt, it is trampled under foot. In the same way, too many Christians lose their “flavor” and become good for nothing.

i. “Most salt in the ancient world derived from salt marshes or the like, rather than by evaporation of salt water, and therefore contained many impurities. The actual salt, being more soluble than the impurities, could be leached out, leaving a residue so dilute it was of little worth.” (Carson)