An ancient Midrash (Mechilta, Yitro, Ba-hodesh, 8) teaches that a person who violates Shabbat is a person who denies God's existence. That made good sense in the ancient world, and even now, for those with an Orthodox worldview: God made the world and commanded us to observe Shabbat. Not observing Shabbat, then, is a slap in the face to God. Imagine that I built you a house, and said that I'd let you live in it for free, so long as you kept one bedroom set aside for me to use when I come to visit. Then, when I visit, I see that you've converted it into a home office - how incredibly rude! The only reason that we would every treat God this way, the thinking goes, is if we don't believe that God really created the world. It's denying God's dominion over, and ownership of, the world. That's why Shabbat violation is such a major transgression in traditional Judaism.
Of course, as liberal Jews, our understanding of God and the world is very different. Most of us don't believe that God literally created the world, as it says in the Torah. And so, it doesn't necessarily follow, with the same directness, that violating Shabbat is denying God.
So, given that, the question arises: what does it mean, as a modern, Reform Jew, to say that "violating Shabbat is denying God's existance"? What does it mean to violate (or observe) Shabbat and, more importantly, how does that tie in so directly with God's reality? I don't know that the question has an answer, but I suspect that trying to answer it will tell us a lot about what it means to be a Reform believer, and a Reform Shabbat observer.