He also promised to take us unto Him as a nation and to become our G-d. This happened at Sinai, accompanied by thunder and lightning, fire and smoke, the ever-increasing sound of the Shofar, and more so with our hearing G-d speaking first to Moshe and then to all the People. His reiteration of the promise at Sinai can be summed up with the pasuk (Sh'mot 19:6); And you will be to Me a kingdom of Kohanim and a holy Nation. So too, our promise to do all that He commands. Our bonding with G-d and He with us is symbolized - more, demonstrated, encapsulated - by Shabbat.
Shabbat started out belonging to all people. Our first introduction to Shabbat is as the conclusion of Creation of the world. As such, we would expect Shabbat to be reflected in all cultures of the world. And to an extent, it is. Other religions have the concept of six days and a special seventh day, which- ever day of the week that happens to be. The world recognizes this aspect of G-d's creation by having a 7-day week. The week is the only unit of time that is not based on nature per se, or divisions and multiples of the natural units of time - day, month, year.
But Shabbat became much more to us. When we next meet Shabbat, it is in the context of the Manna, the bread from heaven that fed the Jewish People. G-d's Shabbat was personalized for us. It became a symbol of the special relationship that was developing between G-d and His people - as He had promised.
Next comes Revelation at Sinai. The formal commands to sanctify Shabbat and not to desecrate it were given to us - the People of Israel. Later we will see the Shabbat in the context of our day- to-day lives, and related to the building of the Mikdash. BEINI UVEIN BNEI YISRA'EL...