The book of Hebrews was written to racially Jewish Christians. So, you would expect to find a lot of Old Testament references in the book, and you'd be right. Let me share one from Hebrews chapter 4.
You may remember the story of the Exodus. Moses was leading the Israelites from slavery to the Promised Land, a land flowing with milk and honey, a land of rest from slavery. Moses and the people approached the Promised Land and so Moses sent spies into the land (Number 13). The twelve spies went and came back with a report. They said that the Promised Land was as good as promised, however all but Joshua and Caleb thought that the people in the land were too tough for the Israelites to defeat. The people rebelled against Moses and wanted to go back to slavery in Egypt. So, because of their unbelief God made them wander in the desert for forty years until that unbelieving generation died. That unbelieving generation was not allowed to enter the Promised Land.
The writer of Hebrews reminds the readers of this story to make sure that they don't repeat their own history. As George Santayana famously said, "Those who cannot remember the past are condemned to repeat it."
"Therefore, since the promise of entering His rest still stands, let us be careful (*fear) that none of you be found to have fallen short of it. 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 it did not combine it with faith." Hebrews 4:1-2 (*'fear' added because practically only the NIV softens the language to 'be careful')
"Let us, therefore, make every effort to enter that rest, so that no one will fall by following their example of disobedience." Hebrews 4:11
The Israelites of that unbelieving generation didn't enter the rest of the Promised Land because of unbelief. Their inability to believe in God's promise kept them from enjoying God's promise. The very same people that saw the 10 Plagues, the parting of the Red Sea, the pillar of fire and that ate the manna and quail from heaven failed to believe God could complete His promise. The same God that had already defeated an army of the most powerful nation on earth was by their side, yet they lacked belief.
The Israelites lacked the belief necessary to enter the Promised Land and the writer of Hebrews warns us of the same thing. Our unbelief can keep us from enjoying God's rest so we work hard to overcome unbelief in our life.
The same God who parted the Red Sea and raised Himself from the dead is by our side, yet we lack belief sometimes.
"...without faith it is impossible to please God..." Hebrews 11:6
We must work hard to maintain our faith. We must work hard to enter God's rest.
Here's the good news.
First, the rest God offers us is so much more complete than the Promised Land. The Promised Land was a prototype, if you will, for what is to come. Our Promised Land is the New Heaven and New Earth. Our Promised Land is a land of perfection where we can once again walk with God and live in eternal bliss. This land of rest is more than worth any effort we need to make.
Second, the work of staying faithful is done with God's power not ours. We must make an effort to stay faithful to Jesus, yes. Staying faithful to Jesus is a community but it isn't something we do with our own power. God gives us the mercy and grace to stay faithful to Him. The saints will persevere but it won't be by sitting on their butts on the sideline. We will stay believing Jesus like the Israelites could have stayed believing God, by going into battle sure of victory.
And when we find ourselves struggling to believe, and we will, let do as the writer of Hebrews encourages us to do:
"Let us then approach the throne of grace with confidence, so that we may receive mercy and grace to help us in our time of need." Hebrews 4:16
Heaven is wonderful. Entering God's rest is transcendent. Let us work hard to enter that rest today.