Ancient spiritual authors were fascinated with this verse. The great medieval Bible commentator, Fr. Nicholas of Gorran, O.P. (+1295) wrote:
For what is salt? It sterilizes soil, seasons food, cures meat, prevents putrefaction. It is made from fire and water. It is offered in every sacrifice. Similarly, the example of a holy life tempers worldly desires, rendering them unfruitful. It adds spice to holy longings, it cures the flesh by mortification, it keeps our desires from rotting. Holiness comes from tears of devotion and fire of love heated in the furnace of repentance. It is offered when it guides and moderates every work.
The 14th-century Carthusian Ludolph of Saxony observes that salt can lose its savor “on account of fear of persecution or adversity, or love of pleasures and wealth, or pride and vainglory, or the desires of the flesh, or neglect of responsibilities, or erroneous doctrines imparted by word or example.”
An intriguing question is posed by Jesuit Scripture scholar Fr. Cornelius a Lapide (+1637): “Why does Christ call his Apostles the salt of the earth rather than the gold, or silver, or precious stones of the earth?” His answer: because salt is a thing universally useful and necessary. That is what Jesus Christ is saying about you.
Through the image of salt, Jesus wishes to pass on to his disciples “the meaning of their mission and their witness. Wisdom sums up in itself the beneficial effects of salt and light: in fact, disciples of the Lord are called to give a new ‘taste’ to the world and to keep it from corruption with the wisdom of God. Christians can diffuse the true wisdom that gives meaning to human life and action” (Pope Benedict XVI).
For “Christians,” writes Erasmo Leiva-Merikakis, “are to raise the level of flavor of every human activity and thus transform it. A thing is wisest when it is most fully itself, when it tastes most like itself, in keeping with its nature. It is ‘foolish’ when it forgets to be what it is.”
A priest I live with was talking at breakfast about parishioners he has known who went out of their way to make their profession their ministry. One was the owner of a shoe store who, upon spying a customer eyeing a certain pair of shoes—but passing on them because of the price—would nonchalantly mention to the shopper that that particular pair of shoes had just “gone on sale.” Another was a florist who secretly supplied flowers to a funeral home for any family who could not afford flowers for a funeral. Even the man’s own daughter was unaware of this life-long mission of her father until she learned about it at her father’s own funeral. These people are the salt of the earth.
St. Catherine of Siena gives us this encouragement:
With tremendous desire my soul longs for you to be so seasoned with the salt of true knowledge and wisdom that you may zealously drive sin and demons from people’s darkened souls. [You can do this] by conscientiously uniting yourself ever more closely with the true light and wisdom, fire and warmth, of divine charity, revealed to us in God’s union with humanity. Start being brave about everything. Don’t look at your weakness, but realize that in Christ crucified you can do everything.