"It still remains that some will enter that rest, and those who formerly had the gospel preached to them did not go in, because of their disobedience. Therefore God again set a certain day, calling it Today, when a long time later he spoke through David, as was said before: 'Today, if you hear his voice, do not harden your hearts.'" NIV translation
It is sad to say but not everyone will be saved. Entering that rest (salvation) requires us to let go of our own work and to trust in the work of Jesus.
"For if Joshua had given them rest, God would not have spoken later about another day. There remains, then, a Sabbath-rest for the people of God; for anyone who enters God's rest also rests from his own work, just as God did from his. Let us, therefore, make every effort to enter that rest, so that no one will fall by following their example of disobedience." NIV translation
Joshua led Israel into the promised land that God had provided but they still looked to their works to give them salvation rest. They would strictly keep the Sabbath and the rest of the law but it did not have the power to bring them to a place where they could rest in the knowledge that they are saved forever.
There is much debate today about the Sabbath and keeping the Sabbath. There are those who strictly hold to the fact that Saturday was the Jewish Sabbath while many "churches" portray Sunday as the "new Sabbath". The simple fact of the matter is that, for those who are in Christ, the Sabbath-rest is every day. Every day we rest in the fact that the work of our salvation was finished when Jesus said "It is finished" while He was on the cross. We also have the rest of knowing that it is permanent and that God takes care of His children. We may not live in the land of Canaan but He still provides for the daily needs of His people.
"For the word of God is living and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart. Nothing in all creation is hidden from god's sight. Everything is uncovered and laid bare before the eyes of him to whom we must give account." NIV translation
The word of God whether spoken (as in the time of Moses and Joshua) or written as we have today is a tool to show men that we cannot be "good enough". It points us to the need for our Savior as there is no sin that is missed and we all fall short of the standard of perfection displayed in His word. Everyone will give an account of their actions. Those that are disobedient and try to be saved through their own works will be judged by the standard of the Word. They will fall short and deserve punishment which is being sent out of the presence of God and into Hell. Those that accept what Jesus has done (and is doing) on their behalf will be saved and live with Him forever.
It is also important to note that, as we see in this passage, it is the Word that judges people and not us. Too many times, the word has been used to beat people up instead of being simply stated and trusted to do its job.