God rests from his labor because He has completed it (kalah, used in Genesis 2:1-2). He rests because there’s no more to do. Yahweh’s rest is satisfaction in work done, work that He pronounces “very good.” The rest God enters isn’t simply cessation, but satisfaction, happiness, completion.
Reformed theologians sometimes speak of the Sabbath as a creation ordinance, as if everyone from Adam to Moses was keeping, or supposed to keep, Sabbath. There’s no biblical evidence for that.
On the contrary, Yahweh alone enters into Sabbath on the seventh day of the creation week. His work is complete, and so He rests. As part of His rest, He pronounces the world good. He passes judgment on it, and blesses it, makes it fruitful. Day is the period of light; light is necessary for judgment; the Sabbath day is the day of days because on it Yahweh pronounces everything good. Sabbath is the day of Yahweh’s enthronement as Judge of all things. It’s judgment day, the “day of Yahweh.”
Adam was created to share in God’s rule of all things, to grow into kingship, to have his senses trained to judge good and evil. But Adam sinned and was cast out of the garden. That was an expulsion from the place of Yahweh’s enthronement, and an expulsion from Yahweh’s rest. Between Adam and Moses, no one kept Sabbath because Yahweh had not opened up His rest to include human beings.
This gives the command to cease from labor a theological foundation. The premise of the command is that there’s an analogy between divine work and human work, and between divine rest and human rest. That says something important about God: He is a laborer, not merely a being of leisure, like the Greek gods. Mary Lefkowitz writes that Hesiod’s Works and Days “tells myths that explain why Zeus has made life hard for humans and why they must work to survive while the gods live at their ease, free from cares.” At least among ancient philosophers, manual labor was regarded with some contempt. But the Bible shows that God was the first manual laborer, planting a garden and digging in the ground to form Adam from the adamah.
But the obverse is as remarkable: If the Bible assumes an analogy between divine and human work, it also assumes an analogy between divine and human leisure. In fact, “analogy” is too weak to capture the point. The Sabbath is the Lord’s day of ceasing, and Israel enters into that ceasing. Israel enters into the Creator’s own enjoyment of creation.
As Uberto Cassuto put it, the Sabbath is the day “on which man rises above the need for hard work that he is called to do on other days for his liveliness, and thereby becomes like God, who rested and was refreshed after the creation of the world” (Exodus, 245).
This shows that the Fourth Word states a privilege and elevation for Israel. The Fourth Word is a word of invitation to Yahweh’s new-Adamic son. Having brought Israel from bondage in Egypt, Yahweh gives them a share in His own rest, His own rule, His own judgment. Israel is elevated to share in the light of Yahweh’s day.
The Fourth Word is an invitation to share in Yahweh’s delight in creation, His joy. Barth said that “an infallible criterion of Sabbath observance [is] whether and how sincerely we are in a position to celebrate it as a true day of joy.”