The book of Hebrews has an unknown author. Many in the past have thought that the Apostle Paul wrote it, or maybe Barnabas or Apollos. In recent centuries, since the Reformation, most have concluded that we just do not know who wrote it. Nevertheless, the Church, under the guidance of the Holy Spirit has accepted the canonicity and inspiration of Hebrews.
It appears to have been written mainly to Hebrew or Jewish Christians, who under persecution were thinking of abandoning their Christian faith and reverting back to Judaism. The writer exhorts them to hold fast to the truth of Jesus Christ and not to forsake their faith in him.
Hebrews was written prior to the fall of Jerusalem in A.D. 70, since this event is not referenced.
The theme of the book is the superiority of Jesus Christ. He is superior to angels, to Moses, to the Aaronic priesthood, and to the Mosaic sacrifices. Jesus is God's last word to humankind for salvation. Strong warnings against the consequences of apostasy are given in both chapters 6 and 10. Well known is the roll call of the heroes of faith in chapter 11. These Old Testament saints are examples of faith for us to follow.
The book closes with practical exhortations to live out the Christian life in faith and practical service.