We as Christians live in the Sabbath every day. The writer of Hebrews employs the term "Sabbath rest" to describe our lives in the kingdom of God, and adds, "Whoever has entered God's rest has also rested from his works as God did from his." (Hebrews 4:10).
In Philippians 4:3-7, the apostle Paul says that he, as a Pharisee, was blameless according to the law - he never consciously committed a single sin, and so, as far as the law was concerned, he was morally perfect. Yet Jesus, in Matthew 5:17-20, taught, "Unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven." I can only imagine the crickets chirping when he said that. How do you exceed - not just match, but surpass - the righteousness of someone who had never consciously sinned? How do you be more perfect than someone who is perfect?
The answer, obviously, is: "You don't." There is simply no way you can do it. Whatever it is that gets you into the kingdom of heaven, it can't be something that you do for yourself; it has to be something that is done to you and for you. This is the doctrine of salvation by grace when you believe. Essentially, you get saved when you recognize that all your good deeds wouldn't be good enough to get you into heaven, so you stop trying, and trust Jesus' righteousness instead. This is a beautiful doctrine of free salvation that is utterly alien to all other religions of the world that stress human effort and exertion.
So why does the writer of Hebrews, in 4:11, say, "Let us therefore strive to enter that rest"? I thought you didn't enter the kingdom of God by striving?
I repeat: you don't. But think about what it means to grow up in the United States in 2014. You are bombarded - literally - by messages from television and movies, people hawking worldviews, trying to tell you what to believe, what will make you happy, what couples do to each other's bodies outside the confines of marriage. You are assaulted by messages in popular music telling you that nothing in life matters as much as having a good time with your friends, possibly with the aid of huge quantities of alcohol, possibly culminating in a trip under the covers with a woman or three. And all of this happens within a culture that believes you can get to heaven by being "good," that believes different people can have different ideas about right and wrong and still all be correct somehow, and that laughs in scorn at the idea that Jesus is the only way to heaven. Many people can't understand why anyone would trust such an old book; many others believe that science has made it impossible to trust the Bible; many others simply don't care about living a good life as long as they're enjoying themselves; many others aren't interested in any rules except the ones they make for themselves.
And this is in one of the most "Christian" countries on planet earth.
You can see pretty clearly that you don't enter the kingdom of God by striving to do good works, but embracing the message of grace means cutting through years or decades of static and false claims made by the society around you. It means consciously rejecting the worldviews and priorities that everyone else around you holds sacred. It means understanding that you will be forever viewed as weird, if you are even making the slightest attempt to live the way Jesus lived, because people in this culture are not used to seeing that kind of lifestyle. You will be mocked by some, misunderstood by others, labeled, stereotyped, rejected, pigeonholed, and ignored. Wrapping your mind around all of that certainly counts as "striving." Intentionally making up your mind to be different and weird certainly counts as "striving." Rejecting all the lies you've been told, even if you're the only one believing the truth, certainly counts as "striving."
This is why the writer says, "Strive to enter that rest." Don't be content to believe what the culture is hawking. Don't be content to aimlessly drift through life. Don't just believe what celebrities tell you to believe, wear what celebrities tell you to wear, sing what celebrities tell you to sing. Seek truth. Ask the hard questions. Find out whether life really has meaning or whether we're all just hanging on till the next weekend of partying. All of that searching is hard work - but if you do it, you will find the restful and relaxing message that, once you find the kingdom, your work is done, and all that remains is to believe.
Jesus said it himself. People asked him, "What must we do, to be doing the works of God?" He answered, "This is the work of God: that you believe in him whom he has sent" (John 6:28-29). The people of his day faced a similar dilemma. The Pharisees told them that righteousness meant following the law, the same law that Paul said he never broke before he stopped trying and started following Jesus instead. Embracing Jesus' message, for the Jews of his day, meant not only rejecting what they had always been taught, but also risking the wrath of these religious leaders who also had powerful political support. Their lives, and their families' lives, might be in danger. They would be cut off from their entire support system, since their society was built around religion and the temple. It is in no way out of line to say that they had to strive to enter the kingdom - not striving by doing good works, but striving to believe the truth of free grace.
Ultimately salvation by grace is revolutionary. No other religion on earth teaches that God himself wants to be in a relationship with you so much that he himself came down here to make it happen and that he demands nothing out of you except that you have faith in what he did when he was here. Yet it is hard work, of a sort, to wrap your head around the simplicity of that idea, especially in a culture that wants you to believe everything except the truth. You can see how accepting Jesus' promise of rest and ease, even though it is welcome, still takes a certain measure of striving, of consciously rejecting other ideas and worldviews, before we can reach the point of being able to accept it.