The answer to this question gets to the heart of the book of Hebrews and what separates the New Testament from the Old. The verse in question opens the book of Hebrews:
“Long ago, at many times and in many ways, God spoke to our fathers by the prophets, but in these last days he has spoken to us by his Son” (Hebrews 1:1–2a, emphasis added)
Jesus is the Ultimate Word from God
While idols often have mouths but cannot speak (Psalm 115:1–8; Jeremiah 10:1–10), we worship the Creator of the universe who does speak and has chosen to reveal Himself progressively to sinful humanity.
God first revealed Himself through the prophets, using many of them to write the Old Testament Scriptures. God’s revelation to and through the prophets was fragmented, incomplete, varied, and anticipated a greater revelation. That greater revelation has come through Jesus Christ (see Luke 24:44; John 5:39–40; 1 Peter 1:10–11).
Jesus Christ is more than a prophet; He is God’s Son, True and better Israel, and the prophesied King who will reign on David’s throne forever. When Jesus opens His mouth, God speaks, for Jesus is fully God and fully man. Jesus the Son shares the ultimate revelation of God the Father through His perfect life, authoritative teaching, atoning death, and victorious resurrection.
All of history led up to the incarnation of Jesus Christ, and the rest of history flows from it, until its culmination when Jesus returns “to save those who are eagerly waiting for Him” (Hebrews 9:28). The new covenant ushered in by Jesus makes the old obsolete (Hebrews 8:13; Jeremiah 31:31–34). Thus, the revelation we have in Jesus Christ is final and definitive. No other revelation is needed; no greater revelation is possible. We don’t need prophets, priests, or kings like in Old Testament times because we have the ultimate Prophet, Priest, and King, Jesus Christ.
To underline this point, the author to the Hebrews continues in 1:2b–4, describing the identity of God’s Son:
whom he appointed the heir of all things, through whom also he created the world. He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. After making purification for sins, he sat down at the right hand of the Majesty on high, having become as much superior to angels as the name he has inherited is more excellent than theirs.
When you receive ultimate revelation from the Creator and Sustainer of the universe, why look elsewhere?
Why We Need This Message
The fact that God now speaks through His Son is incredibly relevant for our world filled with high-profile deconversions and an increasing cultural pressure to abandon the gospel. Turning away from Christ to what is easy, popular, safe, or comfortable may seem desirable in the short run, but doing so rejects God’s ultimate revelation and the only sacrifice for our sins.
When things get tough, we must press on with our eyes fixed on Jesus “so that [we] may not grow weary or fainthearted” (Hebrews 12:3). He is the risen Lord and the Great Shepherd of the sheep who will equip us with everything good to do His will (Hebrews 13:20–21).
We also must not look for special experiences or extra revelation from God beyond what He has given (the Old and New Testaments). Instead, we need to go deeper into who Jesus is and what He means for humanity. We need to know the New Testament that explains God’s revelation in Jesus and its implications for the church and the world. We need to belong to a community of believers who encourage one another to hold fast to God’s definitive word through Christ and help each other grow in faith and holiness as we await our future inheritance (see Hebrews 10:24–25). Ultimately, we need to believe what God says through Jesus.
Postscript: Does this mean we don’t need the Old Testament?
A natural question arises from understanding that God speaks through His Son: Does that mean we can abandon the Old Testament?
Scripture’s answer is an emphatic NO! To dismiss the Old Testament would be a great error, for all of Scripture “is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness” (2 Timothy 3:16). (It’s worth noting that Paul specifically referred to the Old Testament when he wrote those words.)
Instead of rejecting the Old Testament, we need to understand how it anticipates the significance of Christ in the New. Without this understanding, our appreciation for all we have in Christ is shallow at best, and we pass on the Spirit-inspired riches of the Old Testament that are still for our instruction today (1 Corinthians 10:11; Romans 15:4).
 This is why the author of Hebrews often introduces Old Testament quotations with verbs that mention God speaking rather than the typical Scriptural formulation of “It is written” (see Hebrews 1:6, 3:7, 4:7, etc.)
 The book of Hebrews is peppered with commands to not fall away: “[W]e must pay much closer attention to what we have heard, lest we drift away from it” (2:1); “Today, if you hear his voice, do not harden your hearts” (3:7; quoting Psalm 95:7); “Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God.” (3:12); “Therefore, while the promise of entering his rest still stands, let us fear lest any of you should seem to have failed to reach it” (4:1); “Therefore do not throw away your confidence, which has a great reward” (10:35); and “See that you do not refuse him who is speaking” (12:25).