Salt was a highly valuable commodity in hot countries; it still is an essential mineral.  It is life-sustaining and urgently needs to be replaced when it is lost in body sweat.  It was so valuable and essential that salt was part of a Roman soldier's salary … indeed, the word 'salary' comes from the Latin for salt: 'sal'.  Salt was to be included with grain offerings (Leviticus 2:13) and was also used in cooking, cleaning, purification and preservation.  It was important for wound healing, disinfecting toilet waste and preserving food such as fish and meat.  And it is still highly effective for many of those purposes.  

In those days, salt was either collected after the evaporation of salty water near the sea or mined as 'rock salt'.  Both of these would have lots of other minerals in addition to sodium chloride (which is the refined active ingredient in salt).  However, sodium chloride is the most soluble of all those minerals, and so if a pile of salt is exposed to moisture (dew, rain or flood) for any length of time, the potent sodium chloride dissolves out and cannot serve any function.  Then it's only use is in mending roads.

The phrase 'salt of the earth' has become embedded into the English language.  It now means ‘a good person who is reliable, hardworking, kind and generous’.  But that was not really what Jesus meant.  Jesus said that the people in His kingdom would be like active, potent salt – able to make the world clean and preserve what is good.  They would do that, not just by being good and hard-working, but by telling and living the gospel – exposing people to the truth and grace of Christ.  And they would be used by God all over the earth.  However, if they fell away from the Lord Jesus and lost their confidence in the gospel, then they would be useless to His kingdom and people would walk all over them.

God's Kingdom is full of 'salt' – people who are very valuable to Him.  They are potent with truth (Colossians 4:6); their presence can restrain wickedness and preserve what is true; they can clean what is filthy and add God's wholesome flavour to life.  If you are a believer in Jesus Christ, then your purpose is to be salt to bring out the best in the world around you for God's sake, and to stand against sinfulness.  You are designed to preserve God's truth in a deceitful world and heal those who have been wounded by their own sin and the wickedness of others.  So, live up to your calling.  Do not let the world drain your potency for Jesus; and do not stagnate as a Christian allowing the goodness of Christ to leak away from you.  Be potent salt for Jesus' sake.