angelic - There is no need for this verse to be an intimidating one for you. Could I offer an explanation from an in-depth study class I once attended that covered this point from the Sermon on the Mount?
When Christ spoke these words, He was addressing a JEWISH audience who knew exactly what Christ meant by calling them the "SALT of the earth". It was an especially meaningful metaphor for an Israelite. (In the phrase "salt of the EARTH", the Greek word for "earth" is "GE" - which usually refers to the LAND of Israel - not the world at large.)
Let me quote a verse that will explain better what I mean. Go to Leviticus 2:13, where it talks about the tabernacle offerings that were to be given by the Israelites in the Old Testament. "And 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: with all thine offerings thou shalt offer salt." Salt was to be included with every sacrifice, as a symbol of the enduring covenant God had made with His people. In giving an offering that was accompanied by the "salt of the covenant", the Israelites were reminded of God's faithfulness, and were also reminded of their own responsibility to be faithful in their obedience to the covenant God had made with them, to have no other gods besides Him, and to obey His laws.
As a natural preservative and purifier, salt provided the perfect symbol of God's enduring promises. What Christ was doing in Matthew 5:13 was reminding His Israelite audience of the "covenant of salt" God had once made with His people under the Old Covenant.
All during His 3 1/2 years of ministry on earth, Christ was showing those of His own people the way of the NEW Covenant He would make with the children of faith. The "salt" of the OLD Covenant was losing its savour, and there was no way to restore its strength again. Once the New Covenant would be established in Christ's blood, the Old Covenant would become obsolete, and was destined to be "cast out and to be trodden under feet of men." Paul called the Old Covenant "the weak and beggarly elements" (Gal. 4:9). Christ also called the Old Covenant the "old wineskins" that would need to be replaced by new wineskins, so that the new wine (the children of faith under the New Covenant) could be preserved (Matt. 9:17).
He was "taking away the first" (the Old Covenant), so that He could "establish the second" (the New Covenant), which was based upon "better promises" (Heb. 8:6). Instead of putting salt on top of Old Covenant sacrifices, Christ told his disciples to "have salt *IN* YOURSELVES..." (Mark 9:50). If salt is sprinkled IN us, then we ourselves become the "living sacrifices" under God's NEW Covenant of salt. His law is now written on our hearts instead of on tables of stone. As Romans 12:1 puts it, "I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a LIVING SACRIFICE, holy, acceptable unto God, which is your reasonable service."
So, angelic, this verse should not in any way be understood that Christ will "cast you out" and have people stomp on you if you aren't "salty" enough. That meaning is not intended AT ALL by this Matthew 5:13 verse. We are not God's "foster children", being moved from one foster home to another if we irritate the foster parents; we are ADOPTED children. Once a child of faith is adopted into God's family, we are sealed until the day of the redemption of our bodies. This security for the adopted child of God is found all the way back in Ecclesiastes 3:14 "I know that, whatsoever God doeth, it shall be for ever: nothing can be put to it, nor anything taken from it: and God doeth it that men should fear before him." This tells me that the natural response for a true adopted child of faith is not to presume they have a license to sin afterward; instead, the response is that "men should fear before him".
"Hope this helps, angelic :-)