God's Gospel of salvation is indestructible. Jesus isn't saying that salt loses its saltiness. But what He is saying is that men lose their faith. Salt doesn't lose its saltiness but men do lose their vision of the Kingdom and the urgency of its message. This happens when we start tasting like the world.
Salt does nothing to salt, but it can do something for meat or beans.
Therefore, God's people need to be different from the world to live and function as salt. Conclusion God has made Christians into the salt of the earth.