If there is only one gospel, which gospel was being preached in Luke 9:6 which preceded Luke 18:31-34.
The same gospel. Jesus was preaching different aspects of the same gospel in both Luke 9:6 and Luke 18:31-34
We've got some explaining to do...
As the OP correctly assessed there is only one Gospel as one might gather from Paul:
6 I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— 7 not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. 8 But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. 9 As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed. -Galatians 1:6-9
As far as Paul is concerned anyone who preaches a different Gospel from the gospel of Christ is to be accursed. So it's highly unlikely that Paul was teaching a gospel any different than what Jesus taught.
So why the confusion about which gospel is which?
I can identify two closely related fundamental reasons why various accounts of the gospel seem so confusing to most readers:
- We thought we could get a simple, precise, and definitive soundbite summary of the gospel from Paul in 1 Corinthians 15:3-4.
- Consequently, we didn't really know what the gospel was in the first place.
Don't get me wrong. I'm not saying people don't know about the good news regarding the salvation that has been made available through Jesus Christ. Everyone has varying degrees of understanding about that.
It's that we didn't really know how to identify the gospel in its entirety - the big picture that Jesus and Paul both had in mind when they spoke of the same gospel in varying ways from varying vantage points.
What Paul actually says 1 Corinthians 15:3-4,
1 Corinthians 15:1-8 1 Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand, 2 and by which you are being saved, if you hold fast to the word I preached to you—unless you believed in vain.
3 For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, 4 that he was buried, that he was raised on the third day in accordance with the Scriptures, 5 and that he appeared to Cephas, then to the twelve. 6 Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. 7 Then he appeared to James, then to all the apostles. 8 Last of all, as to one untimely born, he appeared also to me.
What Paul actually says in the beginning of verse 3 could be translated more literally as "For I delivered to you among firsts (or among things of first importance) what I also received..."
This is a noteworthy distinction. Instead of Paul describing the following facts as THE most important with respect to the gospel, Paul states that the following facts that he's about to list are AMONG the things that are most important. Paul was not trying to provide a succinct summary of the Gospel as he implicitly acknowledges that there are other things that also rank first in importance. Instead, he is specifically highlighting this chain of events not simply because they are absolutely necessary for there to be any good news at all, but because they directly contradict the specific misconceptions and heresy regarding the bodily resurrection he intended to address in the rest of chapter 15.
Now if Christ is proclaimed as raised from the dead, how can some of you say that there is no resurrection of the dead? - 1 Corinthians 15:12
Again it's not about providing a soundbite summary. He wanted to remind the Corinthians about the Gospel in this specific and critical regard so he could proceed to disabuse them of their nonsense as he had done throughout the first letter to the Corinthians.
So how is the Gospel described as a whole?
We all know gospel means good news and as we already saw in the Galatians chapter 1 verse 7, Paul described it as the "gospel of Christ." Being that he referred to it as the gospel of Christ and not of Paul perhaps we should let Christ explain to us what this good news is all about.:
but he said to them, “I must preach the good news of the kingdom of God to the other towns as well; for I was sent for this purpose.” -Luke 4:43
This is how Jesus ultimately describes the gospel: as good news about the kingdom of God. Indeed announcing the good news of the kingdom of God is integral to his very purpose on Earth and all that he said and did leading up to his crucifixion. If we take Jesus seriously and truly want to have a handle on what the gospel is, we need to try to understand it from within the kingdom framework with which he understood it.
Ok, so what's the good news of the kingdom of God?
If I had to pick one verse then to summarize the gospel of the kingdom of God in its entirety, according to Jesus, it would be from the Lord's Prayer:
9 ...“Our Father in heaven,
hallowed be your name.
10 Your kingdom come,
your will be done, on earth as it is in heaven.
In verse 10 Jesus seems to be employing Hebrew parallelism where what he says is so nice, he had to say it twice, but in a different way so as to drive the point home. For God's kingdom to come, to Jesus, is for God's will to be done on Earth as it is in Heaven.
Think about what that means for the fullness of God's will to do be done on Earth just as it is in heaven: No sin. No pain. No sickness. No death. What would be required to bring this about would be nothing less than a new creation: A complete spiritual and physical renovation of the cosmic order.
This is exactly what is described at the end of Revelation:
Revelation 21:1-4 1 Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more. 2 And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. 3 And I heard a loud voice from the throne saying, “Behold, the dwelling place of God is with man. He will dwell with them, and they will be his people, and God himself will be with them as their God. 4 He will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away.”
5 And he who was seated on the throne said, “Behold, I am making all things new...” - Revelation 21:1-5
Consider the last kingdom of God
In the garden of Eden where God created man in his image as a biological class of ruling representatives over all of God's creation.
27 So God created man in his own image,
in the image of God he created him;
male and female he created them.
28 And God blessed them. And God said to them, “Be fruitful and multiply and fill the earth and subdue it, and have dominion over the fish of the sea and over the birds of the heavens and over every living thing that moves on the earth.” -Genesis 1:27-28
That kingdom was lost because God's firstborn human, Adam, disobeyed. So in order to have the good news of a new creation kingdom, God needed a new firstborn human, a new Adam who, through obeying to the point of death, earns the right to rule over God's new creation not just for himself, but also for all his brothers and sisters who will rule the kingdom with him in everlasting glory... judging angels and whatnot (1 Cor 15:20-21, Romans 5:12-20, Phil 2:8-11, Heb 5:7-10, 2 Tim 2:12a, Rom 8:16-18, 1 Cor 6:3)
So the gospel is a pretty big narrative including a lot of moving parts, but they all fit inside this New Creation Kingdom concept, where God rescues the creation (Rom 8:19-23) by raising up his perfect and now proven image and, in the process, lift up his formerly fallen human images who will all rule his creation in righteousness and obedience unlike before.
19 For the creation waits with eager longing for the revealing of the sons of God. 20 For the creation was subjected to futility, not willingly, but because of him who subjected it, in hope 21 that the creation itself will be set free from its bondage to corruption and obtain the freedom of the glory of the children of God. 22 For we know that the whole creation has been groaning together in the pains of childbirth until now. 23 And not only the creation, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for adoption as sons, the redemption of our bodies. 24 For in this hope we were saved. -Romans 8:19-24
Because the perfectly faithful Christ has ultimate rule over creation, even if one of God's images sins all of creation will not be lost the way it was before due to the sin of the first Adam (not Eve) because the New Adam, Jesus, never sins.
So scoping back down to Luke
In Chapter 18:31-34, Jesus was trying to mentally prepare his disciples for what must be accomplished for him to earn the right to rule God's coming kingdom and blaze a path (as the Way) for humanity from this creation to the next.
In Chapter 9:6, he and his disciples are announcing the reality of God's coming kingdom and preparing the heart of the people for their role in the kingdom for God as co-rulers with Christ where power is given not to those who pursue power and gain in this creation, but to those who are patient and nobly seek out the righteousness of God in the new creation. The healings themselves are a message, a foretaste of God's intention to physically renovate and refresh God's people and the whole world.
When we apply the new creation kingdom framework
to the gospel as discussed in the scriptures, it's easy to understand how all of what Christ did and spoke and what Paul spoke was all about that gospel which is far grander than any story ever told.