God, who at various times and in various ways spoke in time past to the fathers by the prophets, has in these last days spoken to us by His Son, whom He has appointed heir of all things, through whom also He made the worlds (NKJV)
The writer of Hebrews indicates here that God has a history of talking to people. This introduction to the book is remarkable. Why should God, the one described in hymns as "Immortal, invisible, God only wise" and "the Lord of years, the potentate of time, creator of the rolling spheres,
ineffably sublime" communicate with people like our ancestors? How could we deserve even the most cursory understanding of the "Sovereign Being, incomprehensible in His nature, infinite in His essence and perfections, independent in His operations..."?(Charnock)
Nonetheless, this Supreme Creator of the Universe did indeed choose to reveal Himself to humanity, in more ways than one. Theologians break God's revelation of Himself into two categories: General Revelation, which exists as a testimony of general truth about God for all people everywhere (as seen in Psalm 19 and Romans 1),
The heavens declare the glory of God;
And the firmament shows His handiwork.
Day unto day utters speech,
And night unto night reveals knowledge.
There is no speech nor language
Where their voice is not heard.
The New King James Version. (1982). (Ps 19:1–3). Nashville: Thomas Nelson.
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because what may be known of God is manifest in them, for God has shown it to them. For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.
The New King James Version. (1982). (Ro 1:18–21). Nashville: Thomas Nelson.
and Special Revelation, which involves supernatural revelation of specific truth about who God is and His plan for the Universe. The writer of Hebrews here focuses on the latter. At different times and in different ways, God spoke to people through the prophets. This is the claim. Does the rest of the Bible match up with the claim? The Bible is the ultimate collection of Special Revelation, as Peter writes in 2 Peter 1:20 & 21:
knowing this first, that no prophecy of Scripture is of any private interpretation, for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit.
The New King James Version. (1982). (2 Pe 1:20–21). Nashville: Thomas Nelson.
What did these "holy men of God" say? What has God revealed about Himself?
What does the Bible primarily teach?
The Bible primarily teaches what man must believe about God and what God requires of man.