The reference of “salt losing its savor” recorded in Matthew and Luke occurs as part of the Sermon on the Mount (or Plain). Another reference to salt losing its quality is found in Mark 9:50 and most likely is given at another time.
The rationale for seeing Mark’s account as given at a different time relates to the nature of Jesus’ teaching ministry. Often the message given was the same but the places changed such as the “Kingdom is at Hand” proclamation. Of course many of the accounts recorded in the Synoptic Gospels are parallel and given from another perspective when they are not in exact agreement, but not all of the sayings of Jesus were given only once since not all of the disciples were with Him at all occasions and others (who would become part of the 500 who witnessed His resurrected person) needed to hear the same message in different towns. Newspapers and other media did not exist so it should not be surprising that the same teachings were repeated at different times and places.
The Sermon on the Mount starts as a description of the character of Jesus’ disciples (see Lk. 6.20a). Here I reproduce Mt. 5.1-12 since this section defines the “salt of the earth”
“Blessed are the poor in spirit, for the kingdom of heaven belongs to them. “Blessed are those who mourn, for they will be comforted. “Blessed are the meek, for they will inherit the earth. “Blessed are those who hunger and thirst for righteousness, for they will be satisfied. “Blessed are the merciful, for they will be shown mercy. “Blessed are the pure in heart, for they will see God. “Blessed are the peacemakers, for they will be called the children of God. “Blessed are those who are persecuted for righteousness, for the kingdom of heaven belongs to them. “Blessed are you when people insult you and persecute you and say all kinds of evil things about you falsely on account of me. Rejoice and be glad because your reward is great in heaven, for they persecuted the prophets before you in the same way.
Therefore, with seeing these traits, it is easy to see exactly what “the salt of the earth” is. Conversely, also, what losing its “flavor” (or savor, quality) means.
V. 13: “You are the salt of the earth. But if salt loses its flavor, how can it be made salty again? It is no longer good for anything except to be thrown out and trampled on by people.
A note about the word “morantha” (from moraino) translated as “loses its flavor” in both Matthew and Luke’s account given in the “Sermon on the Mount.” In this instance, as I regard the passage, it is a wrong translation. It should read: “become foolish” for these reasons:
1. To translate the word “loses it flavor (lose its saltiness)” is from Mark’s account which I have previously explained was most likely given at a different time than the Matthew and Luke sections. The related content in Mark clearly shows this is the case.
2. In Matthew and Luke Jesus is already using a figure of speech in terming His disciples as “salt”, why would He use a term such as “moraino” which clearly means “to make foolish” as another figure of speech within a figure of speech? No, in this instance, Jesus is clarifying what He means: that the disciples not turn to folly and be characterized opposite of the traits Jesus just described in verses 2-12 of Matthew chapter 5.
3. Jesus was speaking to His disciples to whom He explained figures of speech in instances where they asked. In Lk. 14.35 Jesus warns: ” The one who has ears to hear had better listen!” He wanted the disciples to clearly understand the message to them so He uses: “become foolish”. This is how both Matthew 5.13 and Luke 14.34 should both be translated: “become foolish”.
“You are the salt of the earth. But if salt becomes foolish how can it be made salty again? It is no longer good for anything except to be thrown out and trampled on by people. (v.13)
I. Howard Marshall in his commentary on Luke notes “no attestation” of moraino than “to make folly, become foolish” but still thinks Luke 14.34 should read: “lose its saltiness” (I respectfully disagree for the previously cited reasons). No matter how one translates the word, one thing should be clear: the meaning of “losing its flavor” is “to become foolish.”