“Speak to the children of Israel and say to them: G-d's appointed festivals… For six days labor may be done, and the seventh day is a day of complete rest.:”(Vayikra 23:2)
Hashem is about to teach Moshe the laws of the holidays. Right before it lists the holidays, the Torah mentions Shabbat. Why does the Torah introduce the chapter of the holidays with Shabbat?
There are two steps to emunah (belief). The first step is knowing that the world has a Creator. One has to know that at first there was nothing, until G-d spoke and brought all of reality into being. The second step is to know that G-d is involved in the actual running of the world. Every issue - whether big or small - involves Hashem. He is orchestrating all events at all times.
Shabbat bears testimony to the fact that G-d created heaven and earth, while the holidays, which recall the miracles of the exodus, testify that G-d controls nature on an ongoing basis and can change it at will. By placing Shabbat together with the holidays the Torah is teaching us that both concepts are equally essential to the faith of a Jew. Belief in one step without the other is not considered emunah. It is heretical to think that G-d created the universe, but then "stepped back" and left it to the unfettered laws of nature or in the control of angels or other powers. Emunah is knowing that G-d is the Creator and Sustainer.
Rabbi Issac Farhi