In the past God spoke to our ancestors through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom also he made the universe. The Son is the radiance of God’s glory and the exact representation of his being, sustaining all things by his powerful word. (Hebrews 1:1-3)
In these verses we read of the eternal glory of the Son. Christ is the Creator and Inheritor of the cosmos. He is the Shining-Out of God's goodness and the perfect Image of the Father. It is the Son's powerful word that sustains all things. But then, what about verse 1? Is this saying that Old Testament saints were ignorant of God's eternal Glory - His Creator, Sustainer, Revealer and Heir?
That would be a strange position to take especially since the writer immediately goes on to quote the OT as a consciously Christ-centred Scripture. All his quotations in chapter 1 treat the OT as trinitarian revelation with distinct Persons interacting and speaking in the various Scriptures.
1:5 - Psalm 2 is a conversation between Father and Son
1:5 - David's Seed in 2 Samuel 7:14 is the Son
1:6 - Deuteronomy 32:43 concerns the Son
1:8-9 - Psalm 45 is about God the Son, whose 'God' anoints Him
1:10-12 - The Creator Lord of Psalm 102 is the Son
1:13 - Psalm 110 is a conversation between Father and Son
Later he'll call the OT revelation 'the gospel’ (Heb 4:2) and will hold up Moses as one who "chose disgrace for the sake of Christ." (Heb 11:26).
So even within Hebrews 1 it seems out of the question that the writer intends us to think of the OT saints as ignorant of Christ. How could the OT saints know God without His Eternal Revealer? How could Scriptures like Psalm 45 be read concerning any other than Christ? How could Moses forsake the glories of Egypt and choose disgrace, if he was ignorant of the One who makes such choices worthwhile?
And if we move out of Hebrews, what about Matthew 11:27; John 1:18; 14:6; Colossians 1:15. If Christ is the only revelation of the Father (which these verses insist upon), then how can we deny knowledge of Christ to Old Testament saints? Certainly folks like Justin, Irenaeus, Luther, Calvin, Owen and Edwards were adamant that Christ was known and trusted in all ages by the faithful.
So then, what kind of distinction is Hebrews 1:1-2 trying to make?
Well it's a carefully crafted parallel:
In the past God spoke to our ancestors through the prophets at many times and in various ways, //
but in these last days he has spoken to us by his Son
"In the past" is compared to "in these last days"
"our ancestors" is compared to "us"
"the prophets" is compared to "his Son"
And without doubt something earth-shattering has happened to usher in "these last days." The Son has appeared in a once and for all way (in contrast to the "many and various ways" of the prophetic communications). He came as His own Prophet to address "us" in a way that He didn't address "our ancestors."
In the past, the Son didn't address Israel. He had His prophets do that for Him. Think of Jeremiah 1:
4 The word of the Lord came to me, saying,
5 ‘Before I formed you in the womb I knew you,
before you were born I set you apart;
I appointed you as a prophet to the nations.’
6 ‘Alas, Sovereign Lord,’ I said, ‘I do not know how to speak; I am too young.’
7 But the Lord said to me, ‘Do not say, “I am too young.” You must go to everyone I send you to and say whatever I command you. 8 Do not be afraid of them, for I am with you and will rescue you,’ declares the Lord.
9 Then the Lord reached out his hand and touched my mouth and said to me, ‘I have put my words in your mouth.10 See, today I appoint you over nations and kingdoms to uproot and tear down, to destroy and overthrow, to build and to plant.’
So the Word of the LORD is spoken of as a Person - a Person with things to say. Additionally He speaks and is addressed as a divine Person - He is "of" the LORD and He is the LORD. And this Divine Word of God puts His words into Jeremiah's mouth so that Jeremiah can take the words of the Word to Israel.
That's how things were "in the past." "Our ancestors" (as a rule) didn't get to meet the Son. But occasionally He appeared to the prophets. And the prophets took his words to the nation.
If you were an OT prophet you might have been privileged enough to meet with the Son (like Abraham, Moses, Isaiah etc). But for "our ancestors" that wasn't their experience. They listened to the prophets about the Son and trusted Him through that mediated word.
"In these last days" something monumental has happened. The Son has gotten rid of the prophetic middle-men, just as He's gotten rid of the priestly middle-men (see the rest of Hebrews). Now He has appeared as His own 'Jeremiah' in the flesh. The Son has addressed Israel in person. This once and for all coming has ushered in "the last days".
So it's not that Now we know God through Jesus and before they knew a Christ-less revelation of God. No, Christ is the Representation of God. How could there be Christ-less revelation in the OT?
There is a profound shift between the prophetic age and the arrival of the Son. But that shift is not about Christ becoming Mediator. It's about Him clearing away those temporary middle-men of the Mosaic Covenant. He is the eternal Radiance of God's glory (as Hebrews 1 affirms). Therefore all knowledge of God has always involved Him. As Calvin has said:
“Holy men of old knew God only by beholding Him in His Son as in a mirror. When I say this, I mean that God has never manifested Himself to men in any other way than through the Son, that is, His sole wisdom, light and truth. From this fountain Adam, Noah, Abraham, Isaac, Jacob, and others drank all that they had of heavenly teaching. From the same fountain, all the prophets have also drawn every heavenly oracle that they have given forth. (Institutes IV.8.5)