When day came, Jesus left and went to a secluded place; and the crowds were searching for Him, and came to Him and tried to keep Him from going away from them. But He said to them, “I must preach the kingdom of God to the other cities also, for I was sent for this purpose.”
So He kept on preaching in the synagogues of Judea. Luke 4:42-44
I have often pointed out that Mark tell us in his gospel that Jesus came proclaiming "the gospel" before He had ever been crucified or resurrected and before anyone even knew or comprehended that He would do so. The "gospel" that Jesus preached, according to Mark ,is clearly stated as "the kingdom of God is at hand" (Mark 1:15). With this definition, Luke is in agreement when he quotes Jesus as saying, "I must preach the kingdom of God to the other cities also..." In order to comprehend what Jesus' gospel message is, I think that we have to be able to define "the Kingdom of God" without reference to ideas we gather from a post-cross and resurrection time. The cross and the resurrection are the means to the end, but the end itself is the establishment of the Kingdom of God, that is the good news and not something else. This must be true if those who heard Jesus preaching at this point in His ministry and public career were to make any sense of it. Obviously, if the crowds in Luke's gospel above were eager for Jesus not to leave them, it is because they were excited about what He was doing and saying. They understood the proclamation. For most of us westerners, preaching about the kingdom of God would usually mean one of two things, It would either mean that we are telling people how they can "go" to the kingdom of God, i.e. "heaven" when they die, or we are telling people about how they can become part of the spiritual kingdom of God, the "kingdom" that is "within you". However, neither of those definitions of the kingdom of God can be explained in any pre-crucifixion/resurrection setting. A luxury that was not afforded those who heard Jesus' gospel preaching. The "kingdom of God" must mean something else here.
I submit that the plainest reading of the phrase should be the most obvious definition. Caesar has a kingdom, Herod has a kingdom, and God is moving into their occupied territories in and through the work of Jesus Himself. This was and is the good news! God is becoming King! God's Kingdom is being established on earth as it is in heaven. The time of the Caesars of the world is up and the time for God's world run God's way has arrived. This is the good news of the gospel of Jesus Christ. Of course, there is so much more that needs to be said and worked through regarding this claim, but this is, in it's simplest form, what I believe the gospel is according to Jesus.