John Bright in The Kingdom of God:
The gospel according to Mark begins the story of Jesus' ministry with these significant words: "Jesus came into Galilee, preaching the gospel of God, and saying, "The time is fulfilled, and the Kingdom of God is at hand, repent and believe in the gospel" (1:14-15). Mark thus makes it plain that the burden of Jesus' preaching was to announce the Kingdom of God; that was the central thing with which he was concerned. A reading of the teachings of Jesus as they are found in the gospels only serves to bear this statement out. Everywhere the Kingdom of God was on his lips, and it is always a matter of desperate importance.
Preaching - it is foundational. My only addition to the excellent words of Timothy George (below) is to remember not only was Jesus sent to proclaim the Kingdom of God (as George mentions in Lk 4.43-44a) but that's also what He spoke of during His 40 days here after the resurrection (Acts 1.3).
At the heart of the Christian faith is a Savior who was a preacher. “And Jesus came preaching” (Mark 1:14). This stands in contrast to the gods of Olympus or the deities of the Roman pantheon whose interaction with mortals, when it happened at all, was transient, ephemeral, detached, like a circle touching a tangent. Zeus thundered, but he did not preach. Nor did the dying and rising savior gods of the mystery religions. There were ablutions and incantations and the babbling utterances of the Sibylline Oracles but nothing that could rightly be called a sermon.
But when the divine Logos was made flesh (egeneto sarx, John 1:14), he embraced the full range of human pathos and human discourse: Jesus wept, and Jesus preached. Jesus declared that the very purpose of his mission on earth was to preach: “‘I must proclaim the good news of the kingdom of God to the other towns also, because that is why I was sent.’ And he kept on preaching. . . .” (Luke 4:43-44a).
The old liberal construal of this text was to say that Jesus came preaching the kingdom and what we got was the church. But that way of putting it is to deny the coinherence of the kingdom and the King, a title ascribed to Jesus Christ at several places in the New Testament (see John 12:15, 18:37; 1 Tim. 6:13-16; Rev. 17:14, 19:16).
In the Gospels, Jesus not only proclaimed the kingdom—he was the bearer and the inaugurator of it. This was seen both in what he said—his claim of a unique filial relationship with the heavenly Father (Matt. 11:25-30; John 10:30, 14:11)—and in what he did. He despoiled the reign of Satan through the exorcising of demons, he offered forgiveness to sinners and celebrated the eschatological banquet with them, and he asserted divine moral authority in many ways including the striking “but I say unto you” sayings of the Sermon on the Mount. Thus from the beginning, the content of early Christian preaching was neither a new philosophical worldview nor a code of ethics to improve human behavior, but rather Jesus Christ himself: Jesus remembered in his words and deeds, Jesus crucified, buried, and risen from the dead, and Jesus yet to come again in glory—all of which is included in that earliest of Christian confessions, “Jesus is Lord!”
Next to Jesus, the two greatest exemplars of preaching in the New Testament are John the Baptist and St. Paul. John the Baptist is a liminal prophet who stands at the threshold of the two testaments. In the imagination of the church, John is the one who is always pointing toward Jesus Christ: “Behold, the Lamb of God who takes away the sin of the world” (John 1:29).
This is how Matthias Grünewald presented John in his famous painting of the Isenheim Altarpiece (a copy of which hung above the desk of Karl Barth in his study in Basel). John is standing on one side of the cross with an open book in one hand while he points with the long, bony finger of his other hand at the torturous visage of Jesus on the cross. Of course, we know that John the Baptist had long been dead by the time of Jesus’s crucifixion, beheaded by Herod Antipas. But in the sanctified imagination of Grünewald, he is called back from the dead to make one last appearance in salvation history with the same message he had once delivered during his life on earth. It was a message of negation.
Now this is the testimony of John, when the Jews sent priests and Levites from Jerusalem to ask him, “Who are you?”
He confessed, and did not deny, but confessed, “I am not the Christ.”
And they asked him, “What then? Are you Elijah?”
He said, “I am not.”
“Are you the Prophet?”
And he answered, “No.” (John 1:19-21)
In Grünewald’s painting, in faded red letters in the background, are these words from John 3:30, “He must become greater; I must become less.” From first to last, John the Baptist has a referential ministry and thus serves as a controlling model for Christian proclamation in the early church.
Though Paul became an apostle through his encounter with the risen Christ, we might well reach into the future and drag him back to stand with John the Baptist under the cross, for his own preaching is no less Christologically ordered than that of John. To the Corinthians he wrote, “For what we preach is not ourselves, but Jesus Christ as Lord” (2 Cor. 5:5). Although we know Paul primarily from his letters in the New Testament, he was not called to be a letter writer but rather a preacher of the Gospel, especially to the Gentiles.
I recall Krister Stendahl, one of my former New Testament professors, saying to us that the apostle Paul would have been quite surprised to discover that a few postcards he had dashed off during his missionary travels had made it into the New Testament! Well, Romans is hardly a postcard, and we should not forget that the reading aloud of Paul’s letters in the early Christian communities was itself a form of preaching. But Stendahl’s point still stands: Paul was not a litterateur. He was a preacher who proclaimed the Gospel of Jesus Christ with what the New Testament calls parrhesia, unusual boldness, fearlessness. Paul knew that God had chosen to use the “folly” of preaching to save those who believed, and so, as he wrote to the Corinthians, he was determined “to know nothing among you except Jesus Christ and him crucified” (1 Cor. 2:2).
With Paul’s words ringing in their ears, early Christian proclaimers fanned out across the Roman Empire to engage in what Ephrem the Syrian called “the sweet preaching of the cross.” In doing so, preachers of the early church were not merely expressing their personal opinions or providing entertainment to their listeners. No, they were in the vanguard of the militia Christi, the army of Jesus that sheds no blood. Their preaching propelled redemptive history forward toward the consummation of all things. This is certainly how Matthew 24:14 has been understood, from the age of the apostles right through the dawn of the modern ecumenical movement: “And this Gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come.”
The promise still stands and the task yet remains, for God ever renews his church through new forms of preaching—the martyrs, the monks, the mendicants, the missionaries, the reformers, the awakeners, the pastors and the teachers. Where such proclamation is faithful to the living and written Word of God and enlivened by the Spirit, it is an effective means of grace and a sure sign of the true church.