The book of Hebrews was written at a time in which the nation of Israel had rejected, condemned, and crucified their Messiah, Jesus Christ. Extreme judgment loomed on the horizon and it would not be long before Roman legions destroyed Jerusalem and the temple. Jesus's prophecy that not one stone would be left upon another was fulfilled close to the year AD 70 (Matthew 24:2). God used the writer(s) of the book of Hebrews to send a final warning to the disobedient Jewish nation as the age of law was ending, which also applies to disobedient denominations as the age of grace is ending. This warning has also applied to individual Jews or Christians ever since it was written almost two thousand years ago.
Whenever there has been great apostasy, God has always had a faithful remnant. The book of Hebrews encouraged the faithful remnant among the apostate nation of Israel so that they abandoned Jerusalem and Judea before the ax fell, and they scattered all over the known world preaching the gospel of Jesus Christ. Today, God's faithful remnant is much, much larger. It would be a terrible tragedy if the age of grace were to end like the age of the law. It is true that a great falling away is prophesied prior to the second coming, and even though this is happening, I believe there can simultaneously be a great end-time harvest of souls (of those producing the fruit of the Holy Spirit) and that this is also prophesied.