Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men. (Matthew 5:13)

The savour is the covenant of the Lord. As we enter into a covenant with the Lord, which we do at baptism, we leave the bland throngs of the world, having become salt. In whatever dish of men we enter and mingle, we will add the flavor of love, peace, principle, righteousness, faith, hope, and charity. The world will be blessed for it, and even though there will be slant remarks from those who which they could add flavor, but lack backbone to seek and find the covenants of the fathers, they will respect you and desire that which you bring to the table.

Un-salty salt is useless. Having betrayed the covenant once entered into with the Lord, a person is worse of than s/he was prior to entering into that covenant. In short, where s/he was before slightly mocked, but respected and always desired, s/he will from the day of his/her descent have become utterly useless in the sight of the faithless world. For which cook would add un-salty salt to a dish? The salt would just take up much needed space in the kitchen. Oh, they will drink with you, party with you, and in every way pull you down to their level just to prove to themselves that there is no such thing as a spine! They will lick you up and down to mock the fact that salt is not salty! Don't mistake this for love…

Before the covenant you were not salt. You were judged by the same standards as everyone else. Through the covenant you will be transformed into one of the Chosen Ones, a covenant child of Abraham, Isaac and Jacob. Just as they received a new name of God in the covenant, so will you have a new name awaiting you, as a token of the new life you have begun. You have become "the salt of the earth". This is a wonderful blessing leading to eternal life. But God will not be mocked. He invests everything in you.  Therefore the transformation into salt is irreversible. It's just that the savour of the salt of the betrayer of the covenant becomes tasteless. They become just like useless branches of a fruit-tree, and the people will treat them accordingly:

4 Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me.

5 I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.

6 If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.

7 If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you. (John 15:4-7)