The issue is the Sabbath. God does not stop working on the Sabbath. However, He is not laboring in a steel mill. He is not bending over an engineering table, working on His automobile, or cutting His lawn. What is God doing? Psalm 74:12 says that God is working salvation in all the world, and that work does not stop on the Sabbath.
Jesus is justifying what He did on the Sabbath by the fact that He was doing the same thing God was. He was expending His energy in God's creation, and therefore it was justifiable for Jesus to work. So, creative acts—creative work—of the kind that God is involved in does not stop just because the Sabbath arrives.
The Sabbath is, therefore, an integral part of the same process of Creation that God began on that seventh day. The physical aspect was finished at the end of the sixth day. But the spiritual aspect began with creation of the Sabbath, and it continues to this day, as Jesus establishes in John 5.
In the physical sequence of events—the first six days—God created an environment for man and life. But God shows through the creation of the Sabbath that the life-producing process is not complete with just the physical environment. The Sabbath plays an important role in producing spiritual life. It is life with a dimension that the physical cannot supply. Thus, the Sabbath is not an afterthought of a tremendous Creation. Rather it is a deliberate memorializing of the most enduring thing that man knows—time.
Time plays an important role in God's spiritual creation. It is as if God says, "When this day rolls around, look at what I have made, and consider that I am not finished yet. I am reproducing Myself, and you can be part of My spiritual creation." God created the Sabbath by resting from His physical exertions, thus setting us the example that we must also rest from our physical exertions.
He also blessed and sanctified the day. He did this to no other day! Yet people will argue, even with Christ, that we should not keep it as He did. It is very obvious that He kept it. Yet, it is the commandment that men tend most to disregard as though it is nothing.