In an age before refrigerators, salt was the great preservative. In this memorable metaphor, the Savior calls his disciples salt. They would preserve his teachings and lifestyle among the peoples of the earth, but if they failed in their conscientious following of his example, they would be worthless in his kingdom and would be cast out, as useless salt is cast out.
Salt is plentiful. The evaporation of one cubic mile of seawater leaves about 140 million tons of salts, most of which would be sodium chloride, or common salt. It could also be extracted from the seawaters themselves but with care to remove impurities and poisonous elements in those waters, which are heavy with various minerals.
Salt does not lose its savor with age. Rather, its savor is lost through mixture and contamination. The Lord’s metaphor in this passage may be a warning to avoid any alteration of God-given teachings or any admixture with the philosophies of men or the corrupting influences of those who love evil. The encouragement is for disciples to maintain a pure and undefiled gospel and to season the world with their tasteful living.
In Leviticus, the handbook for Levitical priests, the Lord commanded that “every oblation of thy meat offering shalt thou season with salt; neither shalt thou suffer the salt of the covenant of thy God to be lacking from thy meat offering” (Leviticus 2:13). Salt was a token of the covenant that the Lord had made with his people and was part of Israel’s sacrificial system. That sacrificial system was a type, shadow, and symbol of the great and last sacrifice that Jesus himself would offer (see Hebrews 9–10). Salt ultimately points to the Savior.
Jesus perpetuated the symbol by labeling the people themselves as the possessors and promulgators of his covenant. As the salt would season the meat offering, so the disciples of the Lord Jesus Christ would season the world and preserve his truth in it.