Fausset's Bible Dictionary
An appetizing seasoning of food to man and beast. In the East the vegetable food especially needs salt (Job 6:6; Isaiah 30:24, margin). An antidote to the effects of heat on animal food. A necessary accompaniment of the various altar offerings, bloody and unbloody (Leviticus 2:13, "the salt of the covenant of thy God"; Ezekiel 43:24; Mark 9:49-50). It signifies the imperishableness of Jehovah's love for His people; as an antiseptic salt implies durability, fidelity, purity. The opposite of leaven, the symbol of corruption. Covenants were cemented by feasts and hospitality, the viands of which were seasoned, as all foods, with salt. Hence, "a covenant of salt for ever before the Lord" is an indissoluble covenant (Numbers 18:19; 2 Chronicles 13:5; Ezra 4:14, margin). An Arab who just before would have robbed and murdered you, once you taste his salt, would die to save you; "faithless to salt" is the Persian term for a traitor.
So Jesus, cf6 "have salt in yourselves, and have peace one with another" (Mark 9:50); as no sacrifice to God, and no food to man, is acceptable without salt, so prayers offered without "peace" of heart toward fellow men are savourless; a warning to the disciples who had just been disputing with one another, and judging, fellow men who used Jesus' name though not following the disciples (Mark 9:33-50). Being "salted with the salt of the (heavenly King's) palace," and bound to fidelity to Him, and brought into a covenant of salt with Him, they are called on to have a loving, imperishable savour toward one another and to all men. Colossians 4:6, "let your speech be alway with grace, seasoned with salt," i.e. the savour of fresh spiritual wisdom excluding all "corrupt communication," and tasteless unprofitableness or insipidity (Matthew 5:13; Ephesians 4:29).
Near Colosse was a salt lake, hence the image. The idea in Mark 9:49, cf6 "for every one shall be salted with fire, ,is: the reason why it is better for us to cut off offending members is that the work of every one, believer and unbeliever, shall be tried with fire; to believers "the Refiner's fire" (Malachi 3:3; Matthew 3:11), symbolizing God's searching purity; a consuming fire (Hebrews 12:29) to His foes, who nevertheless shall be imperishable in their doom (salt symbolizing preservation from decay), but purging out only the dross from His people (1 Corinthians 3:13; 1 Peter 1:7; 1 Peter 4:12). The righteous can withstand the fire, for it is part of their present salting as "a living sacrifice" (Isaiah 33:14-15; Romans 12:1). Every offending member and offense must be removed, to enable us to withstand that testing fire and be found without dross unto glory and honour.
The southern shore of the Salt Sea supplied, salt abundantly; compare "the valley of salt" (2 Samuel 8:13) near the mountain of fossil salt, five miles long, the chief source of the salt in the sea. The salt pits (a source of revenue; Josephus Ant. 13:4, section 9) were at the S. of the Dead Sea; the marshes here are coated with salt deposited periodically by the spring rising of the waters which in summer evaporate; and here were the pillars of salt traditionally represented as Lot's wife (Josephus Ant. 1:11, section 4; Apocr. Wisdom of Solomon 10:7). Inferior salt was used for manure (Matthew 5:13; Luke 14:35). Too much salt produced barrenness (Deuteronomy 29:23; Zephaniah 2:9). "Sowing with salt" doomed symbolically to barrenness a destroyed city and depopulated region (Judges 9:45; Psalms 107:34 margin). Salt as expressing purity was the outward sign Elisha used in healing the waters (2 Kings 2:20-21). The Israelites used to rub infants with salt to make the skin dense and firm, and for purification and dedication of them to God (Ezekiel 16:4).
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Fausset, Andrew R. Entry for 'Salt'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/s/salt.html. 1949.