In Matthew 5:13, Jesus tells his disciples that they are the salt of the earth, and that if they lose their salt then they are no longer good for anything. What does it mean to be salt? It means that one's interaction with others is to gospel-filled: one's language, ideas, words, all must be biblical. Two aspects of salt are most apparent:
1. Salt keeps meat from rotting, and so in being salt, we bring the gospel to others to preserve their lives and keep them from perishing.
2. Salt adds flavor, so the conversation is deeper, more nourishing, and gospel-filled.
Now, what are some aspects of being salt to others? A good place to look at gospel-filled speech and interaction is from the first letter of Paul to the Corinthians, chapter one. Here we learn:
1. Salt does not equal cleverness of speech, such cleverness nullifies the cross (1:17).
2. Salt is foolishness to those who are perishing (1:18).
3. Salt has power to those who are being saved (1:18).
4. Simplicity is wiser than academic cleverness (1:20).
5. God has arranged things so that the most clever and "smart" people, the academics, are farthest from knowing God (1:21)
6. The simplicity of "foolishness" of the gospel message--salt--has a saving effect on those who believe (1:21).
7. Christ crucified is a foolish message, a simple message, that makes no sense to the worldly (1:23).
8. This foolishness, in fact, is wiser than the most clever wisdom of the world, because it has power, whereas the wisdom of the world does not (1:25).
9. God actually used unlearned people to preach his message! Not the academics (1:26).
10. Even academics like Saul of Tarsus had to be humbled and unlearn all they they knew in order to be qualified to preach the gospel (1:27).
11. A salty man is a humble man (1:29).
12. Paul was humbled by God in order to proclaim the gospel (2:1).
13. Paul's message is reduced to two things: Jesus Christ and Him Crucified (2:2).
14. Paul came to the Corinthians weak and trembling (2:3).
15. This is key: because of the simple message, his words had no power to persuade in and of themselves, so the converting power of the message was completely on the side of the Holy Spirit (2:4).
16. This key aspect of salt puts all the power in God's hands and not in man's (2:5).
All this has to do with the preserving of men's souls from corruption, from death. This is the preserving aspect of salt. This is the salt that brings unbelievers into a state of believing. It is entirely on the part of God the Holy Spirit that this occurs. Our message is foolish, or simple, and has no power in itself to save. It is a lifeline cast out that may or may not be grasped. The Spirit of God is what turns the heart and draws the believer to the lifeline.
Now, the second aspect of salt, the depth and flavor of conversation--discipleship--is touched on by Paul directly after these verses in chapter two:
1. Among saved Christians, among maturing Christians, deeper things of God are conversed about (2:6).
2. This wisdom is deep, more complex, and "academic," but it is still not the wisdom of the world by any stretch of the imagination (2:6).
3. God's wisdom, his depth, is plumbed by searching the scriptures and unlocking the mysteries of God's doctrine for the continued sanctification of believers (2:7).
4. These mysteries are things the world cannot see. They are "white noise" to the unbelieving, but to the saved, they are rich revelations of truth (2:8).
5. These mysteries are revealed, again, through the Spirit, but now the Spirit dwells inside the hearts of believers (2:10).
Salt means the ability for a disciple to speak a simple, saving message to the world. It also means the ability to share deep and complex doctrines among fellow disciples. The thing both sides of saltiness has in common is that both revolve around Jesus Christ and Him Crucified. The cross of Christ saves the lost. The cross of Christ is also the mystery revealed in all the scriptures.
Now, being aware of this, examine texts like the Sermon on the Mount. Whenever Christ speaks complexly (like 5:13), this verse is meant for disciples, because it contains a mystery to be unlocked. When Christ speaks plainly (like 7:7), he is reaching the unbelievers on the outskirts of the crowd. The verse alone may not be complete for salvation, but it can be grasped easily and draw someone closer to faith. Once you see this distinction, you can find it everywhere in scripture.