Who is the “salt” of the earth?

When Jesus says "you" are the salt of the Earth, does He mean believers or unbelievers? I wonder because nowhere else in the New Testament do we find believers called "salt."

The reference you quoted comes from a discourse from Jesus called the Beatitudes found in Matthew 5:

Matt. 5:11 “Blessed are you when people insult you and persecute you, and falsely say all kinds of evil against you because of Me.
Matt. 5:12 “Rejoice and be glad, for your reward in heaven is great; for in the same way they persecuted the prophets who were before you.
Matt. 5:13 “You are the salt of the earth; but if the salt has become tasteless, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled under foot by men.
Matt. 5:14 “You are the light of the world. A city set on a hill cannot be hidden;
Matt. 5:15 nor does anyone light a lamp and put it under a basket, but on the lampstand, and it gives light to all who are in the house.
Matt. 5:16 “Let your light shine before men in such a way that they may see your good works, and glorify your Father who is in heaven.

When Jesus says a person is to be "salt" of the earth, He means a person is to stand out from the world by giving distinction to God’s truth and thereby bringing life to the world. He uses the metaphor of salt, because salt gives distinction to the taste of food and is a necessary element for sustaining the life of the body.

The passage above is taken from a much longer discourse running from Matthew 5:1–7:27. By the rules of interpretation, we must consider this entire discourse to be spoken to the same audience without interruption unless the text itself indicates that Jesus’ words were taken from different conversations or moment. In this case, we see no such interruption, and therefore the same audience heard all the words Jesus spoke during this discussion.

As we read the entire discourse, we see clearly that Jesus' audience were His disciples gathered around Him, not unbelievers. Therefore, we must conclude that these words were intended to describe the believer, not an unbeliever. Furthermore, the meaning of Jesus' statement only makes sense when applied to a believer. A believer is to stand out from the rest of the unbelieving world, to remain distinct like salt. This is not possible for an unbeliever, since by definition an unbeliever is the world.

Therefore, just as the nation of Israel was called to be a light in their day, likewise the Church is now called to be salt (and light) by bringing God’s truth to a world that does not know Him.