Leviticus 2:13

And every oblation of thy mincha thou shalt salt with salt; thou shalt not withhold the salt of the covenant of thy God from thy mincha, because the twenty and four gifts of the priests are appointed with a covenant of salt; therefore salt shalt thou offer with all thy oblations.

your God’s covenant I have made you party to a covenant, and I have bound you not to offer a worthless sacrifice. Nor may such a sacrifice be eaten: it would be disrespectful.

NEITHER SHALT THOU SUFFER THE SALT OF THE COVENANT OF THY G-D TO BE LACKING FROM THY MEAL-OFFERING. “For a covenant was established with salt as far back as the six days of Creation, for the lower waters [i.e., those of the oceans] were promised that they would be offered upon the altar in the form of salt, and [also as water] at the libation of water, on the Festival of Tabernacles.” (Rejoicing of the Drawing of the Water). This is Rashi’s language, and it is a homiletic exposition of the Sages. Rabbi Abraham ibn Ezra interpreted it in line with the plain meaning of Scripture as follows: “I have brought you into a covenant and made you swear that you would not offer a saltless offering, nor shall it [i.e., a saltless offering] be eaten, because it is a mark of contempt.” Now since salt is the covenant of the offerings, Scripture made this accord the pattern for all such agreements, saying of the gifts given to priests and the dynasty of David that they are [an everlasting] covenant of salt, meaning that they are as everlasting as the covenant of salt of the offerings. There, however, Rabbi Abraham ibn Ezra explained: “A covenant of salt — a covenant decreed, it being of the root, a fruitful land into a salt waste, since a salt waste is as if it has been decreed [upon it that nothing should grow therein].” But there is no sense to his words.
Now it seems to me that since Scripture here states, the covenant of thy G-d, and does not say “the covenant of the Eternal,” which would have been in consonance with the language of the section and the way all the offerings are mentioned [throughout Scripture], or did not say, “the covenant of the Eternal thy G-d” — that the reason for this is because salt is derived from water, and it is through the power of the sun which shines upon it that it becomes salt. Now the nature of water is that it soaks into the earth and makes it bring forth and bud; but after it becomes salt it destroys every place and burns it, that it is not sown, nor beareth. Since a covenant is inclusive of all attributes, water and fire come into it, and unto her shall come the former dominion — the Kingdom of G-d, just like salt which seasons all foods and helps to preserve them, but destroys them when they are over-saturated with it. Thus salt is like the covenant. It is for this reason that Scripture states, Ought ye not to know that the Eternal, the G-d of Israel, gave the kingdom over Israel to David forever, even to him and to his sons by a covenant of salt forever? For this too is the attribute of David. Therefore He says in connection with the offerings, it is an everlasting covenant of salt, for the covenant is “the salt of the world,” and by virtue of it [the world] exists or may be destroyed. I have already taught you to understand from our words in other places the meaning of these three words, brith olam hi (it is a perpetual covenant).

ולא תשבית מלח, “and you must not omit salt;” the reason salt must not be omitted is that it is something that endures, and one of the most important symbolic features of animal sacrifice is to remind the Jewish people as well as individuals of the permanence of G–d’s covenant with us, and that it is not He who has to worry about being transient but we mortals. Seeing that He is not in need of food or drink, this legislation is clearly meant for the benefit of the Jewish people. When a person has sinned, and offered a sacrifice to further his atonement, knowing that after that he will be free of sin, he will be careful not to commit a careless act which resulted in his becoming obligated to cleanse himself from sin. If, on the other hand, he fails to take steps to obtain atonement, he will undoubtedly continue to commit sins as our sages have said in the Talmud tractate Kidushin, folio 20: “once a person has committed a sin-and has not repented and taken steps to rehabilitate himself,-the sin will no longer appear to him as having been a trespass against G–d, but he will treat it as if it had not even been a sin.” They explain this by comparing it to someone who has found a stain on his outer garment. Unless he takes steps to remove it, it will soon become something that he gets used to and before long his whole garment will become soiled. Concerning this subject, Solomon, in Kohelet, 9,8 says: בכל עת יהיו בגדיך לבנים, “be sure that your garments are white at all times;” our sages in the Talmud, tractate Makkot folio 2,add: “do not let them become stained by sin.” The Torah added numerous commandments, which the average person would perform even if the Torah had not specifically commanded him to do so. The reason is that it wished to give us additional opportunities to acquire spiritual merits.