(14) Now after John was put in prison, Jesus came to Galilee, preaching the gospel of the kingdom of God, (15) and saying, "The time is fulfilled, and the kingdom of God is at hand. Repent, and believe in the gospel."
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The gospel Jesus proclaimed is focused on the Kingdom of God. In fact, Jesus' own words bear this out: Only once does He modify the word "gospel," and He does so with the phrase "of the kingdom." In accordance with their Savior's usage of the term, Matthew, Mark, and Luke call it nothing other than this. In several other places, Jesus speaks of "preaching the kingdom of God" to the people.
The Kingdom of God is a huge subject in itself, but its basic meaning whittles down to God's dominion, rule, governance, or realm. In many places in the Gospels, such as His parables (Matthew 13) and the Olivet Prophecy (Matthew 24-25), Jesus points to a future establishment of God's Kingdom on the earth. Notice Matthew 16:27: "For the Son of Man will come in the glory of His Father with His angels, and then He will reward each according to his works."
On the other hand, the Kingdom of God is now ruling over those whom God has called (Colossians 1:13), though they have not yet inherited or entered it fully because they are still flesh (I Corinthians 15:50). The called and chosen await the return of Christ at the last trumpet when He will change their mortal, corrupt, physical bodies into immortal, incorruptible spirit bodies like His (I Corinthians 15:51-52; Philippians 3:20-21; I Thessalonians 4:14-17; I John 3:2).
Thus, the Kingdom of God has both present and future implications. It is a present reality in a spiritual sense for those who believe the gospel in that God rules over them already. As such, they are subject to all the laws and responsibilities being part of God's Kingdom entails. When Christ returns and sets up His government on earth, the gospel will have prepared them for rulership with Him (Revelation 19:7-8). They are presently watching for, praying for, and expecting its fullness at any time (Matthew 24:32-44).
As a future event, the Kingdom of God implies that the gospel concerns itself with prophecy as well. Though many biblical prophecies predicted the coming of Jesus Christ as a Man to die as our Savior, many more prophecies concern His second coming as King of kingsand Lord of lords. The Christian hope revolves around the belief that He will come again, put down all rebellion against Him, grant eternal life to His saints, and establish a Millennium of peace, prosperity, and spiritual growth for surviving humanity. The gospel includes this message of a future utopia.
What we see, then, is that "the gospel of the Kingdom of God" is a general term that covers more than just an announcement of God's Kingdom. It contains the teaching about the soon-coming establishment of God's government on the earth, as well as its present rule over those God has called. It includes instruction for preparing the elect for their responsibilities in His Kingdom, particularly regarding character development in God's image. In a way, "the gospel of the Kingdom of God" is an umbrella term that encompasses the entire revelation of God to man in the Bible. Paul calls this "the whole counsel of God" in Acts 20:27.
— Richard T. Ritenbaugh