You are the salt of the earth; but if the salt loses its flavor, how shall it be seasoned? It is then good for nothing but to be thrown out and trampled underfoot by men.
Matthew 5:13

Salt. It's a seasoning for food. This we well know.

Salt is a preservative. Many cultures have partaken of salt to ratify compacts, because of the symbolism of preservation. In this sense, we preserve the earth from decomposition or destruction.

Salt also refers to wisdom and grace, as in Colossians 4:6:

Let your speech always be with grace, seasoned with salt, that you may know how you ought to answer each one.

Our translation "loses its flavor" comes from the Greek moraino. It was used not only for salt losing flavor, but also a figure of speech for someone being made a fool. You may recognize its root moros, from which we get our word moron.

But consider this: if you are the salt of the earth, then are you not the salt of philosophy? The salt of ethics? The salt of society? The salt of art? The salt of music? The salt of fashion? The salt of science? The salt of the government?

If you are the salt of the earth, then what on earth are you not the salt of?

(Perhaps grammar, because I ended that last sentence with a preposition. Shame on me.)

Then the eleven disciples went away into Galilee, to the mountain which Jesus had appointed for them. When they saw Him, they worshiped Him; but some doubted. And Jesus came and spoke to them, saying, "All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age." Amen.
Matthew 28:16-20

"I will declare the decree:
The LORD has said to Me,
"You are My Son,
Today I have begotten You.
Ask of Me, and I will give You
The nations for Your inheritance,
And the ends of the earth for Your possession.
You shall break them with a rod of iron;
You shall dash them to pieces like a potter's vessel."'
Psalm 2:7-9

I will declare Your name to My brethren;
In the midst of the assembly I will praise You.
You who fear the LORD, praise Him!
All you descendants of Jacob, glorify Him,
And fear Him, all you offspring of Israel!
For He has not despised nor abhorred the affliction of the afflicted;
Nor has He hidden His face from Him;
But when He cried to Him, He heard.

My praise shall be of You in the great assembly;
I will pay My vows before those who fear Him.
The poor shall eat and be satisfied;
Those who seek Him will praise the LORD.
Let your heart live forever!

All the ends of the world
Shall remember and turn to the LORD,
And all the families of the nations
Shall worship before You.
For the kingdom is the LORD's,
And He rules over the nations.

All the prosperous of the earth
Shall eat and worship;
All those who go down to the dust
Shall bow before Him,
Even he who cannot keep himself alive.

A posterity shall serve Him.
It will be recounted of the Lord to the next generation,
They will come and declare His righteousness to a people who will be born,
That He has done this.
Psalm 22:22-31

Are the nations not His inheritance? Are we not His disciples? Has He not told us to teach them to observe all things that He has commanded us? Are we not the salt of the earth?

Will you be salt? Or a good-for-nothing moron?