Answer: In the past God spoke through the prophets, but now He speaks to us by His Son, (verses 1 and 2).
MORE INFORMATION AND/OR OTHER SCRIPTURE REFERENCES:
Matthew 3:17: "And a voice from heaven said, ‘This is My Son, Whom I love; with Him I am well pleased.'"
Matthew 17:5: "While he was still speaking, a bright cloud enveloped them, and a voice from the cloud said, ‘This is My Son, Whom I love; with Him I am well pleased. Listen to Him!'" (Also Mark 9:7; Luke 9:35.)
We cannot be ashamed of the One by Whom God now speaks, nor ashamed of His Words. God commanded His Son what to say and how to say it. Those very Words will judge us at the last day:
Mark 8:38: "If anyone is ashamed of Me [Jesus] and My Words in this adulterous and sinful generation, the Son of Man will be ashamed of him when He comes in His Father's glory with the holy angels."
John 12:47-50: [Jesus said,] "As for the person who hears My Words but does not keep them, I do not judge him. For I did not come to judge the world, but to save it. There is a judge for the one who rejects Me and does not accept My Words; that very Word which I spoke will condemn him at the last day. For I did not speak of My Own accord, but the Father Who sent Me commanded Me what to say and how to say it. I know that His command leads to eternal life. So whatever I say is just what the Father has told Me to say."
Consequences of those who do not listen to the Son and teach not the Word of Jesus, but teach "stories they have made up":
2 Peter 1:17 through 2 Peter 2:3: "For He received honor and glory from God the Father when the voice came to Him from the Majestic Glory, saying, ‘This is my Son, Whom I love; with Him I am well pleased.' We ourselves heard this voice that came from heaven when we were with Him on the sacred mountain. And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts. Above all, you must understand that no prophecy of Scripture came about by the prophet's own interpretation. For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit. But there were also false prophets among the people, just as there will be false teachers among you. They will secretly introduce destructive heresies, even denying the sovereign Lord Who bought them--bringing swift destruction on themselves. Many will follow their shameful ways and will bring the Way of Truth into disrepute. In their greed these teachers will exploit you with stories they have made up. Their condemnation has long been hanging over them, and their destruction has not been sleeping."
