There is only one gospel, the gospel of the Kingdom, i.e. the rule of God where all will be restored again because of the fall. This entails sacrifice and salvation and all the other adjectives used for the gospel. Not sure what the disciples originally understood, but the the death and resurrection of Christ is the means to fix fallen man, namely being born anew as a new creation instead of Adam's fallen one. The object of atoning for sins is not to relish with admiration all thru eternity what was accomplished there (as wonderful as it is) but as a means to continue on. Without it, we would all simply die and everything would end there. It would be beneficial to also point out that promise of Messiah was wrapped up with all the Abrahamic promises, which included a national people and King (Kingdom), so both the cross and Kingdom are part of the same plan.
Jesus Christ was a minister of the circumcision (Israel),
period, Rom 15:8. He was sent ONLY to the lost sheep of the House of Israel,
period, Mt 15:24. With the few Gentiles He talked with, He never preached the Gospel of the Kingdom. Why? Because the earthly Kingdom of Heaven was ONLY for Israel. This is proven in Ac 1:6, when the 11 asked Christ, "Lord, wilt thou at this time restore again the kingdom to Israel?" Christendom tries to smooze this over by saying the 11 were mistaken. However, the 11 had just gone through a 40 days Bible study given by Jesus Christ, concerning the things of the Kingdom, Ac 1:3. The only person in history that knew more about the Kingdom of Heaven, than the 11, was Jesus Christ.
The gospel of the Kingdom that Christ preached during his earthly ministry, that the 12 preached all during Acts, and that Paul preached for the 1st 9 years of Acts, has no part for anyone living today. Nothing that Christ preached during His 3-1/2 years ministry will tell us ANYTHING about our future. Why? Because His ministry was to Israel, not to the Gentiles. We can find out our future ONLY from Paul, the ONLY apostle we Gentiles have, in his latest, most up-to-date epistles, those 7 written after Acts had ended. The information in those was given to Paul by special revelation from Jesus Christ, Eph 3:3. Everything in those 7 epistles was part of a mystery hid in God since the world began, Eph 3:5,9. In this new dispensation, we find that we Gentiles now have a Hope of residing in the uncreated Highest Heaven, the abode of God, where Christ sits at the right hand of God, Eph 1:20 (Christ), Eph 2:6 (Us). In the other 59, 100% Israel books, including Paul's 7 Acts books, no one ever had a Hope of going to the Highest Heaven. The hope in those 59 all-Israel books was either the New Earth or the New Jerusalem, which comes down out of the created starry New Heavens and attaches to the New Earth.
Everything in Paul's Acts Books was based on the OT, Ac 26:22. Nothing in Paul's post-Acts books has ANYTHING to do with the OT, since it all was contained in a mystery before the OT even began. Therefore, it is dishonest to combine the 7 with the 59 (except for Deity truths and salvation). They have absolutely nothing to do with each other. You can't have both, because they contradict each other. Do you want the Highest Heaven or do you want the New Earth? Do you want the Appearing which resurrects you to the Highest Heaven or do you want the all-Israel rapture where you meet the Lord in the air, at His 2nd Coming, at the 7th trump of Revelation, and return with Him to the Earth, like the scripture says?
If you want the Highest Heaven, there is evidence you must give up all those things that were given to Israel, but were never given to the Gentiles, except all of those saved Gentiles during Acts, who were part of Israel, since they were all grafted into Israel, and the only blessings they had were borrowed from Israel.
Here's a short list of those all-Israel must be eliminated from our all-Gentile Doctrine: the Lord's prayer, the Lord's supper, Christ's gospel of the Kingdom, the Great Commission, the churches in the Gospels and Acts (we have a new church in Eph and Col), the New Covenant, the sermon on the mount, all of the Law, water baptism, the Gifts of the Spirit, the idea that you are the seed of Abraham, the New Jerusalem, the idea that we are part of Israel, the idea that Israel are God's chosen people at the present time (Israel was actually set aside in about 63AD), that prophecy is being fulfilled today and that the present nation of Israel was the fulfillment of prophecy, and, of course, the Rapture. And that's only the tip of the iceberg.
None of those things are Truth For Today. Getting rid of all that Israel baggage and finding your Hope and Calling only in Paul's 7 post-Acts epistles, as you should do, is what 2Tim 2:15, RIGHTLY dividing (CORRECTLY cutting) God's Word of Truth is really all about. As it says in 2Tim 2:15, if you do CORRECTLY Cut His Word, you will be approved unto Him and will have no reason to feel ashamed
If you do all of this, you can, one day, be translated during the "Appearing", to the Highest Heaven, where Christ sits at the right hand of God and be part of the "Kingdom of His Dear Son", Col 1:13. Forget about the earthly, all-Israel, Kingdom of Heaven. You can't have it, no matter what you are taught by the mainstream Denominational Churches, who are essentially Jewish Synagogues that teach unobtainable Jewish doctrine to unsuspecting Gentiles. Many are excellent in helping you get saved, which is great, but after that you are better off leaving these non-Biblical churches and having real Bible Studies in a home.
I might mention that the term, "Kingdom of God", is an all-inclusive term that includes all of the future Kingdoms in the both created and uncreated Universe. The Kingdom of Heaven can be called the Kingdom of God (as it is many many times in the Gospels), but the Kingdom of God can't be called the Kingdom of Heaven.
If people would just study their Bible, and believe what it literally says, where it literally says it, instead of trying to spiritualize everything, which is ALWAYS a subjective endeavor, they might develop a real understanding of the Bible. Everything I've said is scriptural.