What is a common theme of early New Testament sermons?

Jason Dohm explains in this video that throughout Scripture, sinners are called to repent, believe the Gospel, and be baptized. Jesus Christ, John the Baptist, and Peter each repeated this common theme in their sermons.

The message of repentance begins in the Old Testament and continues throughout all of Scripture. Repentance is the beginning of Gospel preaching. It is a call to turn away from wickedness and to trust only on Christ.

Acts 3:19 (NKJV) – “Repent therefore and be converted, that your sins may be blotted out, so that times of refreshing may come from the presence of the Lord.”

We're just tracing it through the New Testament. I think we start in Matthew chapter 3 where John the Baptist comes on the scene and his message is, repent for the kingdom of heaven is at hand. And then I think we go to Mark 1 verse 15 where this is Jesus' first sermon at least in the Gospel of Mark and he says the kingdom of heaven is at hand. Repent and believe the gospel. So he and John the Baptist were essentially preaching the same thing.

And then we go to Acts chapter 2 and Peter's preaching on the day of Pentecost and when he finishes that powerful sermon people are coming to him saying what must we do and he says repent and be baptized. So now we have John the Baptist and Jesus and Peter all beginning their preaching the same way with a message of repentance. And then Paul in Acts chapter 20 isn't preaching anything different, he's preaching repentance toward God and faith toward our Lord Jesus Christ. Repentance from the beginning of the Old Testament to the end is the beginning of gospel preaching, a call to turn away from wickedness and trust only in Christ.