“Modern” people often say that Paul (or Peter, or James, or John, etc.) wrote as they did because they came from an old culture. They would say the 2000s represent a different culture; so, we do not have to apply their writings to ourselves in the same way. Behind that issue is the assumption that Paul (or Moses, or David, or Elijah, etc.), not God, was the author of these words. They might even say, "We do not have to accept them as scripture because God did not write them" . In contrast, we believe every page of the Bible is inspired by God; they are all his words.
How did the men that God used to select which scriptures were cannon (like a letter from Paul to the Hebrews)? Was it because a man like Paul wrote it and was obvious to those men that Paul was God's tool, he having lived in the time Jesus actually worked and lived? Maybe it was because of who the authors were: Apostles or "blood relatives" of the Lord Jesus. They might have accepted the book of Hebrews because of the sheer logic and brilliance of the writing.
Paul’s days were days when public discourse was oral. Even written words were not dominant because there was yet no printing press. So, all common discourse tended to be local. Yet the fame of Paul was widespread. If this was not a sign of God's working through Paul, at least it is a sign of the magnitude of his importance. Since his words were copied and recopied there had to be something more behind them than a mere letter of a churchman.
We remind ourselves, as the Bible says, that God was:
- manifest in the flesh
- justified in the spirit
- seen of Angels
- preached to the gentiles
- believed on in the world
- received up into glory
At the time of the events of the Bible God was working in real living people who were doing and saying things among other living people. They wrote it down soon after it happened. God worked among people you could have heard with your own ears and could touch. God is working among men in the church today. Although what they write will not be in the Bible.
A book like Hebrews is "high I cannot attain to it". God gives it to us so that we can grow toward it. A piano teacher knows his student cannot do all that he can, yet he still teachers knowing the student someday will. God gives us Hebrews so that we, someday, will improve to higher Heights.