Quoting from the Psalms, the author to the Hebrews says thus:
"Therefore, as the Holy Spirit says,
'Today, if you hear his voice, do not harden your hearts as is in the rebellion, on the day of testing in the wilderness, where you fathers put me to the test and saw my works for forty years. Therefore I was provoked with that generation, as said, 'They always go astray in their heart; they have not known my ways.' As I swore in my wrath, 'They shall not enter my rest.'" (Heb 3.7-11).
The author of Hebrews goes back the wilderness experience where the people who just left Egypt – by the power of God and through His mighty deliverance – died in the wilderness because of their unbelief and sin. The author now uses this passage from Ps. 95.7-11 as a warning for his hears that they also can be destroyed, left to die in the wilderness of God's wrath. He continues in vv.12-15:
"Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called 'today,' that none of you may be hardened by the deceitfulness of sin. For we share in Christ, if indeed we hold our original confidence firm to the end. As it is said, 'Today, if you hear his voice, do not harden your hearts as in the rebellion.'"
Again, as I read through this book, I am reminded that I am not to sit back and cruise through this life. There is a battle, and there is the chance that I may lose this battle due to apathy. Is it possible that I could fall away from the living God and rebel against him due to a heart of evil? Could I also be deceived by sin and fall away from Jesus Christ? This is what I need to always be aware of and to strive to kill sin and wickedness in my own heart daily. I finish this post with these words from the Author:
"Therefore, while the promise of entering his rest still stands, let us fear lest any of you should seem to have failed to reach it. For good news came to us just as to them, but the message they heard did not benefit them, because they were not united by with those who listened" (Heb. 4.1-2).