“Therefore, salt is good; but if even salt has become tasteless, with what will it be seasoned? It is fit neither for the soil nor for the manure pile; it is thrown out.
“Whoever has ears to hear, let them hear.”
Jesus continues his discourse to the crowd, having instructed them that being his disciple requires loving God more than family, friend, or even one's own life. It requires a willingness to suffer anything - even torture, ridicule, or death. Anyone considering becoming his disciple should consider this before beginning. As a man who begins a tower and does not complete it would be shamed by the community and mocked by a half built monument to his lack of planning, so is one who begins to follow Jesus and does not finish.
Consider the cost. A king would be foolish not to assess the strength of the other army before going into battle. He would estimate the possible cost to his treasure reserve and the lives of his men. If he knows he will lose, he will be willing to compromise and give up money or freedom to avoid annihilation. Anyone desiring to be Jesus' disciple must consider whether he is willing to lose possessions, suffer injury, and even lose his life in the battle. Of course, without becoming his disciple there can be no lasting terms of peace.
Salt has been a valuable preservative since ancient times. It certainly adds flavor, but in the days before refrigeration, salt was one of the best ways to keep meat from turning rancid. In Leviticus 2, God's instructions about bringing offerings included salt. Salt was symbolic of loyalty to the covenant of God, preserving the relationship between God and man. Salt from the Dead Sea contained a gypsum compound that could diminish the effectiveness of salt if not processed correctly. It would lose its taste and become ineffective as a preservative. It could no longer be considered salt. Useless, it would not even be good enough for the compost heap.
In the Sermon on the Mount, Jesus taught that his followers should be salt, so here he clearly refers to those calling themselves his followers. True followers retain their flavor and usefulness. Others may begin with the appearance of saltiness but over time lose their "saltiness". We can seem on the exterior like "salty" believers with all the appearance of holiness. We can focus on doing all the right things without being transformed on the inside. But as the gypsum in the Dead Sea compound eventually degraded the salt to the point that it could not be considered salt, so outward religiosity will eventually show itself to be false. Only the transforming power of the Holy Spirit can truly change our lives.
"The Christian landscape is strewn with the wreckage of derelict, half-built towers, the ruins of those who began to build and were unable to finish. For thousands of people still ignore Christ's warning and undertake to follow Him without first pausing to reflect on the cost of doing so. This is the great scandal of Christendom today, so-called nominal Christianity. In countries to which the Christian civilization has spread, large numbers of people have covered themselves with a decent, but thin veneer of Christianity. They've allowed themselves to become somewhat involved, enough to be respectable, but not enough to be uncomfortable. Their religion, is a great soft cushion. It protects them from the hard unpleasantness of life while changing its place and shape to suit their convenience. No wonder the cynics speak of hypocrites in the church and dismiss religion as escapism."
John Stott - Basic Christianity