(I made a substantial change to this answer since first posting it. I apologize for my initial confusion.]
It is called the Book of Hebrews because the one who titled it recognized something I believe the Holy Spirit is trying to reveal to all the followers of Messiah Jesus today. Namely, that all of us - Jew and "gentile" alike - must be Hebrews! That is, in our human lineage we must all be physical seed of Abraham! Otherwise the letter would not apply to us and would be speaking only to the Jewish followers of Jesus and excluding all the so-called "gentile" followers. Gentile believers would be excluded. And that is an impossibility since everywhere and in every waybook in the authorNT is speaking to the entire body of Messiah. As does every book inThere are no promises just for the NTJews. Or just for the so-called gentile believers. "For all the promises have their amen in Jesus" (2 Cor. 1:20).
This understandingrevelation of our true lineage, although affirmed throughout the letter, is seen most clearly in Hebrews 2:11-16. We are informed that "both he who sanctifies (Jesus) and those who are sanctified (all who are his, both Jew and non-Jew) are "all from one fatherFather" (italicized because not in text); for which reason he is not ashamed to call them brethren." The "one" from whom we arewho is the father of all descendedbelievers is then revealed in verse 16the OT quotes that follow to be none other than "Abraham"our Heavenly Father - not God or Adam,Himself. As "children of God" we are all spiritual brethren. But the text goes even further and gives another reason for Jesus to call us brethren. For we are told he "partook of the same flesh and blood." That "he had to be made like his brethren in all things." In other words we all have the same earthly father as most thinkwell. And he is identified in verse 16 as some Bibles like the NAS imply by capitalizingAbraham. "He [Jesus] only gives help to the added word "Fatherseed of Abraham." This eliminates Adam as the father who gives us our commonality of the flesh that makes us all brothers.
WeIf this seems a bit shocking it's only because we forget that the hope of Abraham was to become "the [physical] father of many gentiles/nations" (Rom. 4:18). That's why God changed his name from Abram (meaning "exalted father) to Abraham (meaning "father of many gentiles/nations"). We know his seed was scattered all over the world when ten of the twelve tribes were exiled by Assyria in 722 BC after God turned them into "gentiles" for their idolatry (see Hos.1. 2 Kings 17:18). So the ones whom the Father leads to Jesus for salvation (John 6:37,44), who are not Jewish, must be that scattered seed. Once this understanding is gained it is readily seen throughout Scripture, unifying the whole Bible into one story about the promises made to one man - Abraham. Galatians 3:29 says it most succinctly: "If you belong to Messiah, you ARE Abraham's seed, heirs according to promise."