You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot.
Though it may seem commonplace today, in the ancient world salt was a precious commodity. Its usefulness was owed to one particular aspect of its nature. It was not its color that made it useful. Plenty of things are white. Nor was its value tied to its texture. Sand is also coarse like salt. It was its saltiness that made it precious. It was useful for both seasoning and preserving food because of its saltiness. If that characteristic was lost, salt was not merely useless, it was a nuisance. Much of the salt in first century Palestine came from the Dead Sea. This was not a very high quality salt and if it got damp it could actually lose its saltiness. At which point it was no longer useful as a seasoning or a preservative. However, salt (even ruined salt) still kills vegetation. This ruined salt could not merely be thrown out, but had to be disposed of carefully in order to avoid harming food sources. Thus, it was often cast on a foot path where it would not harm vegetation but would be ground into the earth.
It is the same with Christians. Our value and effectiveness lies in those things which set us apart from the world. Scripture calls us to be holy like the God we serve is holy (I Peter 1:15). By living a life that is set apart and holy to the Lord, we serve the same purposes as salt. We season the world with our good deeds and we season the gospel of truth with our holy lives. We also preserve the world by our holy living. Our enemy advances like a spoiling mold across the flesh of all humanity tainting the image of God wherever he can. By God's grace and guidance, we fight his advances by shining the light of the truth of the gospel into our world. By Christ's gospel, God reclaims souls from the putrid grasp of the enemy's hand and sets them free to engage in holy living as well.
The application for us is clear. Like the salt of first century Palestine, if we lose that characteristic which sets us apart, we become worse than useless. If we fall back into worldly living and "trample the Son of God underfoot", ignoring the surpassing greatness of His sacrifice, then we became much worse than unusable: we actually bring shame to the cross of Christ (Hebrews 10:29). If we bear the title "Christian" and live like pagans, we only have potential to do damage to Christ's name and all that remains for us is judgment. We are called to be in the world but not of the world. This is no easy task. It is not for the faint of heart or for men pleasers. It is only by the power of God working in our lives that we can accomplish it (Hebrews 13:21). Check your saltiness. Are you living a life set apart for God or are you fading into the background of the world? Remember you serve a Savior who "is able to empathize with your weakness," so turn from sin and call to your Savior for help (Hebrews 4:15).
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