I will wait for the Lord, who is hiding his face from the house of Jacob, and I will hope in him. See, I and the children whom the Lord has given me are signs and portents in Israel from the Lord of hosts, who dwells on Mount Zion. – Isaiah 8:17-18
This Isaiah passage was quoted later on in Hebrews. I statement suggests that the OT is hidden (but still with hope) and also suggesting that the NT would be a time of understanding God. When Hebrews identifies the last days, it would seem that the promises and realization of the Messiah marked that day and a better day coming. All that was hoped for and spoken of in the OT met its end in Christ.
So the message of the NT – summarized well in Colossians —
To them God chose to make known how great among the Gentiles are the riches of the glory of this mystery, which is Christ in you, the hope of glory. – Colossians 1:27
We do not have any hope if we do not have Christ. If Christ is not active in you, and you have not already begun to experience the mystery of His life being lived in you, then, first of all, you are not a follower of Christ, and second, you have no hope — no hope of glory, no hope of fulfillment, no hope of ever being anything, in the final analysis, other than a non-entity, a total zero in life! All this is involved in this vastly important secret.
Long ago God spoke to our ancestors in many and various ways by the prophets, but in these last days he has spoken to us by a Son, whom he appointed heir of all things, through whom he also created the worlds. He is the reflection of God’s glory and the exact imprint of God’s very being, and he sustains all things by his powerful word. – Hebrews 1:1-3