In our study of Hebrews chapter four, we will look at keeping the Sabbath and what it means to rest in the Lord. This chapter will help us to understand what Jesus meant when He said: "It is finished".
"Therefore, since the promise of entering his rest still stands, let us be careful that none of you be found to have fallen short of it." NIV translation
In chapter 3, we saw that Jesus was and is much more than just a "good man" in that he is the Way to enter into the rest which is in God. Paul continues by reminding us that the promise is the same for us as it was for the people of Moses' day. So, what does he mean by being careful and falling short? He is actually saying that we should fear ignorance of the word of God and falling from grace. The entire word of God speaks of the grace that was to come through Jesus Christ. Many people have a misconception of what is meant by "falling from grace" and the simple meaning is, when we attempt to add anything to grace (such as works), we no longer have grace which is a gift.
"For we also have had the gospel preached to us, just as they did; but the message they heard was of no value to them, because those who heard did not combine it with faith." NIV translation
The grace of God through Jesus Christ was not some kind of "plan B" and it has never changed. The good news was shared with the people of Moses' day and they did not believe it and put their trust in God. This is where faith comes into play, in that, we have to have faith that Jesus' sacrifice was sufficient and that, when He said "It is finished", He meant it.
"Now we who have believed enter that rest, just as God has said, 'So I declared on oath in my anger, "They shall never enter my rest."' And yet his work has been finished since the creation of the world. For somewhere he has spoken about the seventh day in these words: 'And on the seventh day God rested from all his work.' And again in the passage above he says, 'They shall never enter my rest.'" NIV translation
We enter this rest of salvation by faith and that has been God's plan from the beginning. When He rested on the seventh day, it was not that God was tired but that His plan (our Way of salvation) had been set in motion.