"There (at Marah) did He provide for it (Israel) Law and there did He test it." (Shemos 15:25). The Sages explain (Sanhedrin 56b) that the "Law" refers to the mitzvos of Shabbos, the establishment of a court system and honoring one's parents. What do these laws have in common? Why did Hashem dictate them at Marah?

The story of Marah is very strange, indeed. B'nei Yisroel had crossed the Sea of Reeds only three days beforehand, beholding unprecedented miracles and salvation, yet suddenly they took to complaining about the bitter ("mar") water. Why did they act in such a manner?

B'nei Yisroel's complaints at Marah were a reflection of their slavery experience. B'nei Yisroel had just been emancipated and, as slaves, had not really borne responsibility for themselves and lacked a normal social envoronment. Hence did they react to the bitter water with gripes, for this is the way of slaves, who do not have a sense of security about their wellbeing and lack social and legal structures within which to redress problems.

Hashem thereupon provided laws that address these needs. Shabbos represents the ultimate authority of Hashem as Creator and Controller of all. Hashem is the Caretaker par excellence, and B'nei Yisroel needed to understand that they were in His secure hands. The court system reflects the legal structure, vesting authority in the Sages to rule and mediate. Honoring one's parents provides a structure of authority within the family.

It was essential for B'nei Yisroel to know that they were always in Hashem's hands, and a proper sense of authority had to be taught. It may be for this reason that the same bitter waters were rendered sweet and potable (rather than new water being provided), for the shortcoming of B'nei Yisroel was in attitude, not in substance, and a change in attitude to reliance on Hashem and proper acceptance of the authority of Moshe were all that was needed.