The Lord uses concrete images in his teachings about the Kingdom of God and about his disciples. One of those He uses in the Sermon on the Mount is that of salt. "You are the salt of the earth" (Matthew 5:13). Why did He say salt of the earth and not salt of the sea? It's because sea salt cannot go bad, lose its flavor, but salt that is mined from the earth can. What He wants to talk about is salt losing its flavor or ability to season.
Disciples of Christ are to be like salt, that is, to enhance the quality of life for those around them by proclaiming and living the truth of the Gospel. Christians can thus transform life in this world and help raise it to a level that gives glory to God. But what if the salt goes bad, becomes insipid? "It is no longer good for anything except to be thrown out," says the Lord (Mt. 5:13). The root of the Greek word that is translated "insipid" or "tasteless" is an interesting one: moros, from which comes the English "moron." It is not only wrong to lose your Christain faith or fervor, it is stupid!
One commentator says that either Christians enhance the quality of human life or they have no reason of being. He even goes so far as to say that the reason that the world is so full of sin and confusion is that Christians are not living up to their vocations. We have become insipid, "stupid" salt. We don't know who we are and what we're supposed to be doing, and so many opportunites are lost for bringing the truth and love of Jesus to others.
If God has chosen us, baptized us into the Holy Trinity, fed us with his word and sacraments, then we have a mission, a responsibility--especially to others who have not been so lavishly blessed. So it may very well be our fault that the world is in its present state. We have to recover whatever degree of our "flavor" that we may have lost through sin, negligence, or indifference. We can still be all that God wants us to be; we can still spiritually enhance the life of the world by our greater faithfulness, fervor, and prayer.
"Have salt in yourselves and be at peace with one another" (Mark 9:50). To be a disciple of Christ is a great, noble, and holy calling. Do we live up to it? Are we making a contribution in the world toward the eternal salvation of souls? Our Christian mission is not necessarily a high-profile one. Salt, like yeast (another image He uses), works in a hidden manner, but it really makes a difference.