I think it pays to make sure we're preaching the same Gospel today that Jesus preached when He was on earth; and that we do so with the same emphasis as His.
Jesus preached the Gospel of the Kingdom of God. Many in Israel were waiting for the Kingdom of God, but Jesus explained the Kingdom of God in a way that surprised many of them.
For starters, Jesus said that His Kingdom is not of earth - it's of heaven. If His Kingdom was of earth, His disciples would have carried arms and attempted to prevent Jesus' arrest by the Roman soldiers. But His Kingdom is not of earth, it's of heaven. Therefore Jesus called the Kingdom of God the Kingdom of heaven.
Jesus also explained that to enter the Kingdom of heaven a person first had to understand it. That must have really weirded some of the Jews out because they were expecting the coming Kingdom to be centred in the city of Jerusalem which anyone could simply walk into, or hop on a donkey and ride into - so why would someone need a spiritual understanding to enter it? It's because, as Jesus explained, the Kingdom does not come observably or to a specific location - instead, the Kingdom is inside you.
Then Jesus told a Jewish leader that a man would never even see the Kingdom of God unless he is born from above. That concept would really have pulled the rug out from underneath some of them, because they thought that having been born as a physical descendant of Abraham was qualification enough to see the Kingdom of God. But Jesus said - you must be born from above to see it. Anyone who travels to Jerusalem can see the physical city of Jerusalem - but as for the Kingdom of heaven: you've got no chance of seeing it unless you're born again.
That tells you - as Paul said - that the true Jerusalem is not the earthly city of Jerusalem - it's the heavenly Jerusalem. And the true place of worship is no longer the earthly Jerusalem - the true worshipers now worship in spirit and in truth - and the Father is looking for such to worship Him. True Jewishness is of the heart - it's not a matter of physical birth, and even Gentiles who believe are included. The true mount Zion is in heaven, not on the earth. The true tabernacle or temple is in heaven, not on the earth. The new Jerusalem is actually people - it is the bride of Christ - for she has made herself ready, and to her it has been given to be clothed in white linen, for the white linen is the righteousnesses of the saints.
Another aspect about the Kingdom which was completely hidden from most of them was that Christ the Messiah first had to be mistreated and suffer and die on the cross for our sins and be buried and be resurrected on the third day, before entering into His glory, into heaven. And the disciples would become witnesses of all these things, and this Gospel of the Kingdom would first be preached to all nations, with various signs accompanying believers, such as: in Jesus' Name they shall cast out demons; they shall speak with tongues; they shall see visions, and prophesy; they shall lay hands on the sick and they shall recover; and nothing shall by any means harm them; and all who believe and are baptized, all who call upon the Name of the Lord, shall be saved - and then the end shall come. All this would happen before the great and dreadful day of the Lord, the day they were all expecting.
Not many of the Jews saw that coming! They had a totally different idea of the nature of the Kingdom. (They thought Messiah would immediately welcome all of the Jews into His Kingdom, with it's capital being Jerusalem.) But this is what the Father has actually been up to ever since the days of John the Baptist until now - He wants the Name of Jesus, and the cross - this good news of the Kingdom - to be preached all over the world. He is drawing all men to Jesus, because Jesus was lifted up on the cross to be the Saviour of all men. The Father is looking for true worshipers, those who now worship not in Jerusalem, nor on mount Zion, but in spirit and in truth.
The parables of Jesus illustrate the true nature of the Kingdom. Messiah would go away to receive His Kingdom - because it's in heaven, not on earth, that He receives His Kingdom. Furthermore, it was almost implied that there would be a long delay which would test the hearts of those who claim to be waiting for His promise. Then it was explained how that the children of the wicked will continue to live alongside the children of the Kingdom in this present world until the time of the end - and then the Father will divide the wicked from the among the righteous and only after that will the righteous - and only the righteous - enter into His Kingdom. Jesus even foretold that the Son of God would be collectively mistreated by physical Israel (to whom the promises were originally given) and that as a result, the Kingdom would instead be given to another nation - which is the Church, comprising of both Jews and Gentiles who believe. The promises originally made to Israel have not failed: the elect have obtained them. The Church is more Jewish than Israel is.
Jesus told His disciples to expect persecution just as He was persecuted - but He comforted them by promising to return soon. His delay does not mean God is slack concerning His promise: it means God is patient, not willing that any should perish, but that all should come to repentance. Besides, a day is with the Lord as a thousand years, and a thousand years as one day. "Soon" is from heaven's perspective - from God's perspective Who dwells in eternity - not from earth's perspective.
But the day of His coming and Kingdom shall come. This is the great, hoped-for salvation that we look for and that we announce. It includes the resurrection of the dead. Christians who have already died won't miss out on that great day because Christ shall bring them with Him when He comes. Then we which are alive and remain shall be caught up together with them in the clouds - and we shall all forever be with the Lord. This is the blessed hope which saints of God have looked forward to and prophesied about since the beginning of time. This is the hope of the Gospel which we preach! This is the good news of the Kingdom: Jesus is coming! There will be new heavens and a new earth, wherein dwelleth righteousness.
In the meantime, men must prepare to enter it, regardless of whether they are Jew or Gentile. God has concluded all under sin, that He may have mercy upon all. God gave us the Way to enter in, which is His Son Jesus. Therefore the message of the cross, of the Name of Jesus, of repentance, and of faith and love are an intrinsic part of the equation, a central part of the message, crucial to a correct understanding of the nature of the Kingdom. God has redeemed us through His blood. That's the part the Jews stumbled over! That's the understanding of the Kingdom which we must preach. That's the true nature and message of the Kingdom: it's all about Christ Jesus and His cross and about the second coming of Christ. The sufferings of this present time as we live for Jesus are not even worthy to be compared with the glory that shall be revealed in us when Christ returns. We know we have access through His blood, therefore we look forward to His coming. God is bringing many sons unto glory: we are heirs, joint-heirs with Christ through faith, which is symbolized by baptism. In the meantime we have the witness of His Spirit in our hearts, we have righteousness, peace and joy in the Holy Ghost. These things are our guarantee of eternal life in His Kingdom when He comes. All men need to hear this.
"Christ in you, the hope of glory."