Matthew 5:13 You are the salt of the earth. But if the salt has become tasteless, with what shall it be salted? It is no longer good for anything except to be cast out and trampled underfoot by men.
When we say that we are salt, it means that we exercise our influence over the earth created by God to keep it in its original condition. The earth, which was created by God, has become fallen. In a sense, it has become rotten and corrupted.
Salt kills the germs and eliminates this rottenness. Any medical doctor can tell you that salt kills germs, eliminates rottenness, and preserves things in their original condition. Hence, the function of salt is to preserve what God has created.
The entire earth is becoming more and more rotten. Therefore, we must exercise our influence over this corrupted earth. To the corrupted earth, the people of the kingdom of the heavens are the element that keeps the earth from being fully corrupted.
For the kingdom people to become tasteless means that they have lost their salting function. They have become the same as the earthly people, with nothing to distinguish them from unbelievers. To become tasteless is to lose the distinction between us and the worldly people.
It is to become the same as the worldly ones. Being the same as the worldly people is the opposite of the nature revealed in [Matthew 5] verses 3 through 12. It means that we are no longer poor in spirit, mourning for the negative situation, meek, hungry and thirsty for righteousness, merciful, pure in seeking God, making peace, willing to be persecuted for the sake of righteousness, and willing to be reproached for Christ.
It means that we live, walk, and behave like the worldly people. If such is our case, we become tasteless, and the salt has lost its function.