God loves a cheerful giver. Those who work frantically in His presence displease Him. Those who believe, those who enter rest, are equipped for evangelical work -- refreshed work.
Therefore, since a promise remains of entering His rest, let us fear lest any of you seem to have come short of it. For indeed the gospel was preached to us as well as to them; but the word which they heard did not profit them, not being mixed with faith in those who heard it. For we who have believed do enter that rest, as He has said: "So I swore in My wrath, they shall not enter My rest,'" although the works were finished from the foundation of the world. For He has spoken in a certain place of the seventh day in this way: "And God rested on the seventh day from all His works"; and again in this place: "They shall not enter My rest." Since therefore it remains that some must enter it, and those to whom it was first preached did not enter because of disobedience, again He designates a certain day, saying in David, "Today," after such a long time, as it has been said: "Today, if you will hear His voice, do not harden your hearts." For if Joshua had given them rest, then He would not afterward have spoken of another day. There remains therefore a rest for the people of God. For he who has entered His rest has himself also ceased from his works as God did from His. Let us therefore be diligent to enter that rest, lest anyone fall according to the same example of disobedience. For the word of God is living and powerful, and sharper than any two-edged sword, piercing even to the division of soul and spirit, and of joints and marrow, and is a discerner of the thoughts and intents of the heart. And there is no creature hidden from His sight, but all things are naked and open to the eyes of Him to whom we must give account (Heb. 4:1-14).
A promise of salvation rest exists as long as it can be called Today. And the professing Christians of the first century were in a very similar situation as the Jews in the wilderness were. Both had heard the gospel, and in both cases some of God's covenant people heard with faith and some did not. In both cases, those who had believed entered God's rest -- exclusion from the rest being the threat given in the psalm. Even though God rested from creation since the sixth day, some through lack of faith do not enter that rest. The initial sabbath rest at creation and the Canaan rest spoken of in Scripture did not exhaust the promise. In effect, they were the promise. The promise extends down throughout all history -- whenever there is a Today. The rest we enter by faith is salvation rest. Christ rested from His work of the new creation just as God rested from His work of creation. But neither profits apart from faith. We must work hard, be diligent, to enter rest. And remember that God sees the heart.
The Jews in the wilderness and the Christians in the first century were parallel peoples. Remember that our author is not contrasting the Jews and Christians -- he compares them to one another. They had the gospel preached to them, just as we have (v. 2). The Christians of the first century who were contemplating a return to the sacrifices of "bulls and goats" were in the same position as those Jews who wanted to return to Egypt. The New Testament draws a broad typological parallel between the history of the Jews in the Old Testament and the people of God in the New. The captivity in Egypt is parallel to the entire Old Covenant era. God brought them all out of the land of Egypt, and they spent forty years in the wilderness. The Church was brought out of her captivity in the resurrection of Jesus, and spent the next forty years in the wilderness. Jesus rose from the dead around 30 A.D. and that forty year period ended in 70 A.D. During that forty years, the Church went through her wilderness testing -- receiving the Word of God, establishing apostolic authority, and so on. And then in 70 A.D. the "invasion of Canaan" began, where the world is now Canaan, and the exploits of the new Joshua are subduing all nations under His feet. This makes the book of Hebrews a New Testament Deuteronomy, given to the people of God at the end of their forty year period, and given as a preparation for how they were going to conduct themselves in the land that the Lord their God was giving to them.
After His conquest of sin and death, the Lord Jesus established a new creation. He was the agent of this second creation, just as He was the agent of the first. In this passage, we see Christ resting. Just as God created all things, and then rested, so Christ recreated all things, and then rested. After each work was complete, the rest was on-going. But living in the midst of that ongoing rest was of no value to those who did not mix this understanding with faith. Consequently, the people of God have the first day on which to worship just as the Jews had the seventh day. But the day is not the center -- Christ is the center. In other words, God created the heavens and earth in six days and then rested. Christ recreated the heavens and earth, making a new heavens and earth, and then rested. He did this in three days and nights -- half the time. When He emerged from the grave on the first day of the week, He was entering His resurrection rest. This is why a "sabbath rest" remains for the people of God.
And this is also the central reason why Christians do not observe the seventh day as their day of rest. But given the "everlasting" language of the Old Testament with regard to the sabbath, nothing short of a new creation could have moved that day from the seventh to the first. The seventh day was imbedded in the created order, and it would stay that way until the created order was overhauled, redone, recreated. Which it was in the resurrection of Jesus.
And His resurrection is the basis of all covenant obedience. We see here, as we see all through Hebrews, the ongoing fact of blessing and curses. This is the nature of covenants. We are to be diligent to enter the rest. Note the cooperative contrast between diligence and resting. We are to rest in the presence of a two-edged sword, which can slice us as thinly as possible. He discerns the thoughts and intentions of the heart. No creature is out of his view, and we must all give an account to Him. And this is the foundation of rest?!
Yes -- God deals with His people by means of covenant. Consequently when we live under Him in a covenantal fashion we are living in the way He made us to live. This does not "work" because it makes sense to us; it works because this is how God made the world.