The Creator & Sustainer
Speak to the children of Israel and say to them: God's appointed festivals… For six days labor may be done, and the seventh day is a day of complete rest. (Lev. 23:2)
God is about to teach Moshe the laws of the festivals. Right before it lists the festivals the Torah mentions Shabbos. Why does the Torah introduce the chapter of festivals with Shabbos?
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 God spoke and brought all of reality into being. The second step is to know that God 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.
Shabbos bears testimony to the fact that God created heaven and earth, while the festivals, which recall the miracles of the exodus, testify that God controls nature on an ongoing basis and can change it at will. By placing Shabbos together with the festivals 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 God 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 God is the Creator and Sustainer.