Jesus is the beginning of the resurrection of the dead, for any 1st century Jew whether they affirmed it or rejected the doctrine this was a national and global event, either marking the beginning or the culmination of the restoration of Israel and Kingdom of God and His rescuing and saving judgment on the world, of full restoration and justice done in the world in and through the resurrection of His people as fulfilled humanity at last, the ones in whom the calling of Adam, renewed in Abraham is completed and world is run and saved through that raised humanity. It is the ultimate affirmation of creation as good, and resurrection is where creational monotheism and the restorative justice of God that comes out of it meets in the eschatological moment.
So Jesus wasn’t just some strange single odd event, a happy ending after the bad event of the crucifixion, or just to so we could ‘go to heaven’ which isn’t something the early Christians and drew from it at all, in fact the wide-spread influence of this two story concept of reality where so many modern Christians just read into the phrase resurrection or the phrase Kingdom of heaven or God to mean going to heaven ‘up there’ and see the goal as leaving this world for some non physical, not material realm somewhere else leaving this world behind is so far from the meaning and worldview in the NT, and from their creational monotheism it misses what is being said entirely and embraces a Platonic and Gnostic attitude towards creation and human physical existence that it is one of great tragedies with Christians for the last two or so centuries. No, in the NT conception and classical Christianity there is a one story universe, that comprises the two spheres of creation that are heaven and earth, and they overlap and move within each other and intersect, so heaven isn’t somewhere out there, somewhere else, it’s right here, their the two parts of creation made for each other, and that will become married together when God is all in all (heavenly Jerusalem comes down to earth, to unite and marry with it), of God’s will to be done on earth as it is in heaven (not heaven as it is in heaven, it’s already done in sphere of creation) God has no intention of abandoning creation or embodied humans whom he declares good, there isn’t a plan B, and this is what the resurrection of the dead is indicating and stating.
And this is the early Christian proclamation, that the looked and hoped for rescue of Israel, forgiveness of sins, the incoming Kingdom of God to Earth, and the rescue and renewal of the world had began, because the resurrection of the dead had began. The global event had began and split in two, the Messiah first as the first-fruits leading the way, and the rest of humanity following Him latter, this is the term and world Paul and the NT is speaking into, and resurrection only meant bodies in the 1st century, to anyone Gentile (who universally denied it), or Jews (some of whom affirmed it), but all understood the term to mean physical bodies coming back to life, the concept of non-physical resurrection is just a category mistake and would seem oxymoronic to 1st century ears, there was a wealth of language that referred to various concepts of non-physical states from the despair of Homeric dead of Hades to the exalted souls that finally escaped the prison of the bodies in Platonic and related beliefs and philosophies, to ideas of astral immortality (all of which affirming the dead stay dead) and among the Jews a wealth of their own language for the intermediate survival prior to resurrection such as soul, spirit or angel (which crops up in Acts), none of which could remotely be confused with resurrection, it was a completely different category, and to bring later blurring and confusing that collapses the two-stage process to how early Christians or Paul himself understood resurrection is anachronistic. It would not be until the late 2nd century that we began to see resurrection as a term used by anyone in a way that doesn’t refer to people being raised physically out beyond death into a new physically, and that was by Gnostic Christians who wished to clothe their views in Christian language and remain within Christian circles (hence the strained attempts to re-appropriate the term resurrection, if only they knew how effectively their views in these areas would largely be embraced in the end, making it rather sadly ironic).
And resurrection was more than just being returned and raised back to this corruptible mortal body, the whole point of the phrase, and the concept of the resurrection of the dead as an event, was that the people of God were being raised out and beyond death, beyond it’s hold or any decay or corruptibility, into an immortal physically (it is life after life after death). The very pronouncement of stating that the resurrection of the dead has began, though unexpectedly as Paul explains now split in two, which Jesus raised first and then us in Him following at His appearing, is stating unequivocally that Jesus is the first of the resurrection, in that He is the first to be raised through and out beyond death and decay, that’s the point, the resurrection of the dead has began, God’s Kingdom has the new creation as come in power to the world, in a way no one expected, being launched in the resurrection of the Messiah, that vindicates Him as Messiah and Lord of the world (this is way in the resurrection narratives the disciples don’t sudden say well that means we going to be raised or even less we are ‘going to heaven’, but rather, Jesus has been raised and has taken authority over the world, therefore we have a job to do). To say Jesus had been resurrected, that He was the first of the resurrection of the dead, in which and through which death was defeated and had no hold on Him, is to say He was raised by God through and out beyond death, into a new physical life that is incorruptible and immortal (hence why He is the first, no one else has been resurrected before, some in the OT narratives and the Lord’s ministry were raised back into corruptible mortal life, to die again, they had not been resurrected self-evidently, they would die again, to say Jesus was resurrected was to say He was raised out beyond death into an immortal physicality), the beginning and launch of the global resurrection of the dead, and the renewal of the whole world. It is the affirmation of creation, of the coming of the hoped for judgement and justice of God and the sweeping away of the decay and corruptibility that afflicts our corrupt bodies and world. Therefore Jesus resurrection both vindicated Him as Messiah and made clear to the disciples, to Paul, that God’s rescue and renewal had began in a way they hadn’t ever expected or foreseen, that the resurrection and new creation had been launched onto the world and Jesus was now the Lord of the world and the earth was under new management.
The first thing to remember with Paul’s letters (or for that matter the Gospel or Acts or Revelation) is they are a whole letter, and single and dense argument, and that any verse of chapter has to be read as part of that whole argument (and particularly looking towards the overall conclusions for where Paul’s thought is going with it). The whole of 1 Corinthians was dealing with various troubles in Corinth, one of which was Gentile Christians slipping back into the pagan denial of resurrection (which was linked with acting with various degrees of immoral behaviour since they believed God’s Kingdom had come in full and they waited to escape these bodies, and what they did now in their bodies didn’t matter, also has links with their hunger for spiritual experiences), and much of the letter is correcting this idea. It particularly enters into discussing the nature of the resurrection and the form of existence and body we shall possess emphasising and basing that it will be like that of the Lord’s (He is the first of the resurrection, the first-fruits and proto-type, what Paul had witnessed there was what he bases his thought and discussion on here), and states it will be a spiritual rather than a natural body. Now here is where modern ears, particularly here in the West mishear this importing ideas into the text that have not no place in first century language and worldview, bodies means bodies, not a spirit (they already had a wealth of language for that, some of which crops up at points in various NT documents themselves such as Peter’s angel when they believed he was dead (note they didn’t think he was resurrected at all) and the Pharisees when Paul is brought before the Sanhedrin and says he is being persecuted because of his belief in the resurrection of the dead causing the dispute (and major area of conflict between Pharisees and Sadducees parties, one that was and is intensely political, resurrection is a revolutionary concept that denies to tyrants their great weapon and helped inspire rebellion) at which point the Pharisees suggest Paul might have seen Jesus’ angel and got confused (note importantly again this a refusal of Paul’s declaration of Jesus’s resurrection or that the resurrection of the dead began with Him, they deny that resurrection, but it also shows very clearly that just as everyone else in the 1st century, everyone had a clear idea what resurrection from the dead was, and what it was not, it involved bodies, and being raised out and beyond death, and that Paul wasn’t making any novel use of the term in this respect), and phrases Paul will use elsewhere such as ‘absent from the body, away with Christ’ , or being in Paradise (a brief resting oasis awaiting the resurrection). Rather, to return to Corinthians, the very use of saying bodies means to any first century ear bodies, physical bodies (to read spiritual bodies as immaterial spirits, would be like say a square circle or a married bachelor in that context, it’s contradictory), anything else is anachronistic, what Paul is talking about what will (and what does in relation to Jesus) animates the resurrection body, in that our bodies and nature will be animated and given life by the Holy Spirit, a Spirit empowered body, rather than our current mortal corruptible bodies animated by psyche or soul, which he develops with the statement that the mortal will put on immortality, the corruptible incorruptible, importantly note we won’t put aside our mortal bodies (or even worse escape from them, that not a Christian idea and denies the goodness of creation) but rather our mortal bodies will be clothed with immortality and brought out beyond death into a body empowered and given life by the Holy Spirit Himself (and that is is the case already with Jesus, to whose resurrection body Paul is a eyewitness to). The difference again is what animates, not what the body is made of, it is the difference between a steam ship as opposed to a sail ship, rather than a steel or wooden ship, it’s what animates and drives the body, not what it’s made of. After all, that would mean when Paul would talk of growing and becoming a spiritual man rather than a natural man, said man would dissolve and become a immaterial ghost O_O, which is clearly not what Paul is saying (and the same applies here, it’s what animates not what the body is made of).
And this whole discussion is framed after discussing Jesus’ resurrection, repeating the tradition he received and handed on, together with the inclusion of himself as a last eyewitness to Jesus resurrection body, also discussion how then could some in Corinth say there was no resurrection of the dead, of which Jesus was the first fruits (which goes on to discuss based on Jesus what that involves and what our bodies will be like), and that if they are right we are in our sins still. This is because as Jesus is the first-fruits of the resurrection of the dead, than in Him the resurrection of the dead has began, and the Kingdom of God launched and the forgiveness of sins given, the return of YHVH to Israel and the end of exile for Israel and humanity and the beginning of the restoration and renewal of creation, and if Jesus was not ressurected (out and beyond death into incorruptible and immortal physicality) than they shown themselves to be liars, the resurrection of the dead has not began, God’s Kingdom has not come and Israel and humanity as not be returned from exile with the promised forgiveness of sins, and they still in their sins, in exile and under death’s power and are liars against God. And in the same part Paul affirms that the Lord has been raised, and in Him comes the resurrection of the dead, and shall all be made alive, first with the Messiah, leading the way, than at His appearing those who belong to Him, when He hands over dominion to God the Father after He has destroyed all dominion, power and authority (drawing on Psalm 2), reigning (He is reigning now, all power on heaven and earth are His, that what the Ascension is about) until all enemies are put under HIs feet and God’s reign is established with death at the last placed under His feet and destroyed at His appearing the raising of those who belong to Him, creation is renewed the decay of the old creation sweep away, and He hands it over to the Father so God is all in all. But is Paul indicates that (rather obviously, I don’t have an immortal body like the Lord’s yet, nor does anyone else that I know of 
) hasn’t happened to anyone but Jesus yet, it will happen to those fallen asleep but that is yet to happen, we and all creation are in the intermediate state between the inauguration and completion of creation and humanity and the defeat and destruction of death began in Christ, and it’s fulfilment at His appearing when God will be all in all.
And because of this (bearing in mind the problems the denial of resurrection of the dead had caused in Corinth, with the default return to pagan denials of resurrection and the subsequent view that only spiritual experiences mattered and therefore what they did in and through the body didn’t matter) Paul completes this section not with Christ has risen and we shall be raised so be relax in that knowledge, but rather turns to the collection of the church in Jerusalem going through a famine, because he has remained then that we should give ourselves to the work of the Lord, because it is not vain. What we do in and through our bodies is not lost or in vain, or who we affect affect the lives of others and their existence (or creation for that matter) because it won’t be lost, but rather the body and every act of love, kindness, justice, of giving to those in need, and of a holy life is not lost but will be taken up when we are raised and made part of the full, transfigured body to come, there is full continuity between our embodied life now and our embodied life then at His appearing (in fact to be fully human is to be embodied, our bodies are fully bound with who we are) the body is for the Lord and Lord is for the body, and is shall be raised at His appearing to be as He is. So what we do in and through our bodies now matters immensely, and what we do with others, and their lives matter immensely, and how we treat creation matters immensely, because of the resurrection and the renewal of creation, in Christ’s Resurrection is the ultimate affirmation of creation, and God’s intention to rescue, restore and bring humanity and creation to completion and defeat and sweep death and decay away, and that this has already began, and in the Spirit we begin to participate in it, and are the ones the Lord works through to see that new creation and resurrection life brought into the world, until He returns and brings all to completion.
So in short, Paul isn’t ambiguous here, it’s only the muddle we have made in collapsing the two stage nature of resurrection into one and confusing the hope of resurrection with a more Platonic creation abandoning concept of ‘going to heaven’ and reading that into Paul’s discussions of resurrection bodies (and Christ’s body) and for that matter Kingdom of heaven (which is God’s Kingdom coming to earth, so that His will is ever more done on earth as it is in heaven, and will be when Jesus appears, destroys death and hands over the Kingdom to the Father). Much of modern Christianity is so focused on ‘saving souls’ when God is interested in saving the whole of the human being and all creation, than many wonder what the resurrection is about and often think it refers just to the idea of going away to heaven.
But to borrow a phrase of Bishop Wright’s, heaven is nice, but isn’t the end of the world, this world is our home, and it will remain so forever, neither it nor our full existence will be left or sweep away but rather taken up and transfigured, empowered by the Holy Spirit in share in the full Life of God through the union with Christ, the first full human and the first the resurrection.