Why Shomer Shabbos?

Observing Shabbat shows one has faith in G-d. It shows that one believes G-d created the world yesh mi'ayin (creatio ex-nihilo). The reason is that by resting on Shabbat we affirm that there is no need to rectify or repair anything. We indicate our faith that even by doing nothing, G-d will provide for us, just as He created an entire world from northing and provided for it. Furthermore, observing Shabbat indicates our faith that G-d is One; there is no other. He commands and we accept His sovereignty; we guard the sanctity of the day He calls holy. In these ways, keeping the laws of Shabbat is an expression of faith.

(Rabbi Chaim Kramer)