The oldest extant Greek manuscript that contains the Bible book of Hebrews is Papyrus 46 (P46), dated to about 200 C.E. and is a divided manuscript, with half of it in the Chester Beatty Library in Dublin, Ireland and the other half in the University of Michigan Papyrus Collection. So many are wanting to discount the Bible at a whim, rather than be willing to dig deeply to establish its authenticity, which has been well established.
If a person were to critically examine the letters the apostle Paul wrote, it can be seen that he concluded all, with the exception of Romans ( in which he concludes with "to God, wise alone, be the glory through Jesus Christ forever. Amen"), with the expression "the underserved kindness of the Lord Jesus Christ....be with all of you" or something very similar.
Paul did not write up to Hebrews 9:14 and then stop, for at this point, there was no conclusion. But more so did Paul continue writing, explaining the Mosaic Law and its various "ordinances of sacred service" and "legal requirements".(Heb 9:1, 10)
Without his explanation, understanding the Mosaic Law covenant and its antitype, the "new covenant"(Luke 22:20), would have been, without question, so much more difficult to grasp. Jesus was noted for using illustrations extensively in his ministry, as a teaching tool, to explain God's kingdom. Paul too, sought to explain features of the Mosaic Law and its meaning, for the Law was a "tutor leading to the Christ".(Gal 3:24)
For example, starting at Hebrews 9:15, Paul explains why Jesus is a "mediator of a new covenant", quoting from Jeremiah 31:31-34 at chapter 10:16, 17 concerning the "new covenant" and God's law being written, not on stone, but in a person's heart. He further noted the "Law has a shadow of the good things to come, but not the very substance of the things."(Heb 10:1)
At Hebrews 11, Paul gives the proper explanation of faith, saying: "Faith is the assured expectation of things hoped for, the evident demonstration of realities though not beheld."(Heb 11:1) He then gives many examples of individuals who showed genuine faith, starting with Abel, then continuing to mention Enoch, Noah, Abraham, Sarah, Isaac, Jacob, Moses, down through the period of the judges, whereby Gideon, Barak, Samson, Jephthah, David as well as Samuel are seen as examples of faith for modern day worshippers of Jehovah God.
At Hebrews 12, Paul begins by saying: "So, then, because we have so great a cloud of witnesses surrounding us, let us also put off every weight and the sin that easily entangles us, and let us run with endurance the race that is set before us, as we look intently at the Chief Agent and Perfecter of our faith, Jesus. For the joy that was set before him he endured a torture stake, despising shame, and has sat down at the right hand of the throne of God."(Heb 12:1, 2) Thus, Paul brings the real meaning of faith "home" by encouraging everyone to "look intently at the Chief Agent and Perfecter of our faith, Jesus."
At Hebrews 13, Paul says: "Remember those who are taking the lead among you, who have spoken the word of God to you, and as you contemplate how [their] conduct turns out imitate [their] faith."(Heb 13:7) He wishes for all to imitate the faith of the "older men" in the congregation, but especially of Jesus Christ.
Just before concluding his letter, he says: "Now I exhort you, brothers, to bear with this word of encouragement, for I have, indeed, composed a letter to you in few words. Take note that our brother Timothy has been released, with whom, if he comes quite soon, I shall see you."(Heb 13:22, 23)
Paul concludes his letter to the Hebrews with the words: "The undeserved kindness be with all of you" (Heb 13:25) as he did twelve of his other letters.