I determined long ago that Paul wrote the epistle to the Hebrews because of internal evidence, which I believe abounds. I am not about to detail all that evidence here because my posts I believe are not generally read by members here. It would take too much of my time for the fruit it would bear.
There are several factors that enter into the determination of the author of this excellent letter. I may just give one or two.
Lu 13:1 There were present at that season some that told him of the Galilaeans, whose blood Pilate had mingled with their sacrifices.
2 And Jesus answering said unto them, Suppose ye that these Galilaeans were sinners above all the Galilaeans, because they suffered such things?
3 I tell you, Nay: but, except ye repent, ye shall all likewise perish.
4 Or those eighteen, upon whom the tower in Siloam fell, and slew them, think ye that they were sinners above all men that dwelt in Jerusalem?
5 I tell you, Nay: but, except ye repent, ye shall all likewise perish.
6 ¶ He spake also this parable; A certain man had a fig tree planted in his vineyard; and he came and sought fruit thereon, and found none.
7 Then said he unto the dresser of his vineyard, Behold, these three years I come seeking fruit on this fig tree, and find none: cut it down; why cumbereth it the ground?
8 And he answering said unto him, Lord, let it alone this year also, till I shall dig about it, and dung it:
9 And if it bear fruit, well: and if not, then after that thou shalt cut it down.
This is a probationary time for consideration for reflection and repentance of former thought and actions of Israel concerning Jesus Christ. God uses 40 year segments for this purpose because he is kind and pitiful, and long suffering , and forgiving. But when something like a fig tree is cut down, the possibility of fruit bearing has ended and a removal is needed.
There were 40 years between the crucifixion by the nation of Israel and the subsequent resurrection of Jesus Christ and the nations dispersion from the land. The connection is made with this generation of Jews and the time under Moses in the wilderness in Heb 3 with a warning that this same scenario has already played out in their history once before and without their repentance will again for the second time.
So, the letter to the Hebrews is written by someone who has a deep passion and love for this people that he will do all he can that they do not perish as a people in great judgement that is very soon to come to pass. Paul warns that they are nearing the end of the probationary period and they have not repented and if they do not repent, having been shown the light of truth and even interacting with the Holy Ghost, there would be no remedy of God for the generation but judgement and rejection, which came to pass in about 3 years or so after the book was written. The destruction of the city of the great King, Jerusalem was destroyed in 70 AD. This is essentially the "generation of Jesus Christ" (Mt 1:1 Ps 90:10)
Why is this important in determining who is the author of Hebrews. It is because the love of God for his people Israel is expressed through the person of the apostle Paul particularly. Consider what Paul says here.
Col 1:24 Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body’s sake, which is the church:
Would someone read that within the context and understand what a profound statement that is and compare it to Hebrews. I could expound on it here for a while but no one would care and no one would believe me.
An important development for this people Israel took place after 7 years of the forty when the Jewish rulers rejected the preaching of the gospel and they persecuted the preachers, and killed Stephen. It wasn't until then that we are introduced to Saul, who would become the apostle Paul. This is important timing of God because the call for Israel to repent and be born again, every one of them, was a legitimate promise of God that required only what they were told, repent and believe the gospel of Jesus Christ, the son of God. Had they done that there would have not been a ministry of Paul in the same way and these "mysteries of God" given to the apostle Paul to preach to this age would have remained hidden and history would have unfolded differently.
This brings me to the main point of this post, internal evidence that Paul wrote this letter as if he were Jesus Christ himself, which Col 1:24 affirms in type. There were only three persons in the history of Israel that had such an abounding love for Israel and their salvation that they would willingly suffer the damnation of God for their sins that they may be forgiven. Only three, and two only could stand as a type of that kind of loving sacrifice but only one with the capability of doing it and coming back from the dead.
Ex 32:31 And Moses returned unto the LORD, and said, Oh, this people have sinned a great sin, and have made them gods of gold.
32 Yet now, if thou wilt forgive their sin — ; and if not, blot me, I pray thee, out of thy book which thou hast written.
Acts 3:22 - 7:37
Ro 9:1 I (Paul) say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost,
2 That I have great heaviness and continual sorrow in my heart.
3 For I could wish that myself were accursed from Christ for my brethren, my kinsmen according to the flesh:
4 Who are Israelites;
Mr 10:45 For even the Son of man came not to be ministered unto, but to minister, and to give his life a ransom for many.
Joh 15:13 Greater love hath no man than this, that a man lay down his life for his friends.
Ro 5:8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
Joh 10:15 As the Father knoweth me, even so know I the Father: and I lay down my life for the sheep (the sheep in scripture is Israel).
Joh 10:17 Therefore doth my Father love me, because I lay down my life, that I might take it again.
Though this is not proof that Paul was the author, the question would be, why wouldn't he be?
He wrote;
Ro 10:1 Brethren, my heart’s desire and prayer to God for Israel is, that they might be saved.
Amen, Brother Paul, it is my prayer too.