Contemporary English Version (CEV)
I heard a great sermon the other day on God's rest. In summary, God rested on the 7th day after he had created the heavens and the earth. He rested because everything had been done. When we are commanded to rest, we are placing our trust in God that everything has been done. We don't have to complete God's work, and we don't have to figure out what God didn't do and make up for it. Our rest is a sign of complete confidence in God that everything that needs to be finished has been accomplished, and there is nothing for us to add to God's work.
So when the Israelites complained their way across the wilderness, they were proclaiming that God hadn't done enough. When we grow anxious about God and our salvation, we are stating that what God has done in Jesus Christ might not be enough. When we try and be perfect to make up for our faults, we are living as though Christ's sacrifice on the cross wasn't perfect enough, and we have to finish this work.
What God has done is enough. May we trust in God and rest in confidence.