Parashat Vayera introduces us to the differences between Avraham and Sodom.
The differences that eventually the Chachamim of the Mishna, in Pirkei Avot, would name and define for generations as to who is considered a descendant of Avraham, and what are the characteristics of Sodom.
Already in the opening of the Parasha we find Avraham's tent wide open for guests and Sara in her ripe old age, rushing to prepare cakes.
In Sodom, on the other hand, the fists are clenched, as tzedaka is prohibited by law. There were many other wicked cities and kingdoms in the world, but Sodom was the only one where the wickedness was legal.
This factor makes Sodom the exact opposite of Yerushalayim, which was called the city of justice, "She was full of justice, righteousness lodged in her." Sodom is punished with salt and brimstone, and this was an eye for an eye. Just like the people of Sodom were taught to only take and receive, so too they were punished by salt whose quality is to absorb everything in to itself.
This will cause the Prophet Yishayahu to compare Zion to Sodom, "we have become like Sodom, we are similar to Gomorrah."
Also the children of Avraham upon who it's written in our Parasha that "he'll command his children to follow the path of Hashem, to do righteousness and justice," might fall in to the pit of selfishness.
We therefore must cling to the ways of our fathers, to teach righteousness and justice and only place a little bit of salt on our tables.
Salt on our tables comes from the commandment in the Torah of "you shall salt all your offerings with salt."
A custom which is supposed to remind us while offering a Korban, that we are not, heaven forbid, giving something to Hashem, who possesses the whole world, rather are receiving, and therefore we add salt, the symbol of any giving.
