There is a clear shift in language: the word "dead"
appeared 11 times in v.1-34, only 3 times in v.35-58; the word "body"
appeared 10 times in v.35-58 but not once in v.1-34.
The analogy of the seeds stresses both continuity and transformation; one living thing, through death, can have two modes of existence. The analogy is similar to Jn 12:24, the necessity of death for fruit; out of death a new expression of life springs forth. It is possible for the dead to rise again, as their own experience of sowing grain gives evidence.
The life that comes forth does so in a transformed "body". It matters not whether one speaks of "wheat" or any other grain.
The kind of resurrected body is ultimately decided by God as he pleases; and what pleases him is to "transform our lowly bodies so that they will be like Christ's glorious body." (Php 3:21)
In the anaology of bodies, Paul tries to point out the great variety and kinds of "bodies" there are in the world, all of which is God's work in creation.
The word "spendour" anticipates v.43 where the resurrected (heavenly) body has "glory".
Continuing the analogy of seeds in v.36-38: the body that is sown and raised is the same body yet not the same, as it is to be raised imperishable, glorious, and powerful.
The present body is described as shameful or "in dishonour" for its lowly state in comparison with its glorified state.
"Natural" describes the present earthly state (belonging to the life of the present age) and "spiritual" describes the future heavenly state (belonging to the life of the Spirit in the age to come). It is "spiritual", not in the sense of "immaterial" but of "supernatural". The transformed body is not composed of "spirit"; it is a body that is under the ultimate dominance of the Spirit.
Paul cites Gen 2:7 ("the man became a living being") followed by an extension of the original verse; the addition of the words "first" and "Adam" is designed to lead to the second line.
The word "being" (Greek psyche) is the noun for the word "natural" (Greek psychikos) in v.44; the word "Spirit" (Greek pneuma meaning "God breathed") is the noun for the word "spiritual" (Greek pneumatikos) in v.44. Both verses refer to the natural body and the spiritual body.
Contrasts between Adam and Christ:
Adam and Christ are representative of those who belong to them. Since believers have all shared the existence of the first Adam, they are being called to bear the image of the last Adam. They are being urged to conform to the life of the "man of heaven" in character and behaviour. (Possibly, "so shall we" is originally "let us also" which is found in more manuscripts.)
Transformation of the body is absolutely necessary because both the flesh and blood (the living) and the perishable (the dead) are subject to weakness, decay, and death and are ill-suited for the life of the future.
"Mystery": once hidden but has now been revealed through Christ. Paul wrote about the same event in 1Th 4:13-18. The word "we" simply means he is currently among the living.
Both the living and the dead must be transformed instantaneously at the second coming of Christ. "The last trumpet" does not mean that no more trumpet will be heard; it may simply be a signal for the End, something like sounding the last battle cry (Jer 51:27), warning of the approaching day of judgment (Joel 2:1), announcing the coming of the Lord (Zec 9:14), summoning the people to God from the four corners (Is 27:13).
Paul declares the abolition of death by citing two OT texts; both were kept with its OT context but they vary from the Septuagint. Is 25:8 "the Lord will swallow up death forever".
Hos 13:14 "Where, O death, are your plagues? Where, O grave, is your destruction (penalty)?"
Although the abolition death is accomplished with the resurrection of believers in the future, it is expressed in the present tense because the beginning of the End has "already" set in motion.
Paul's theology on the relationship of sin and the law to death: they together have brought death to all mankind. Sin is the deadly poison that has led to death. The law is what gives sin its power as it reveals the depth of one's depravity and rebellion against God. Both sin and the law have already been overcome in the cross.
Despite Paul's misgivings over the Corinthians' theology and behaviour, and despite their generally anti-Paul stance on so many issues, from Paul's point of view they are ever his "dear brothers and sisters". Paul's exhortation: on the negative side, do not "shift from the hope of the gospel which you heard" (Col 1:23); on the positive side, engage in activities that are specifically Christian, or specifically in the interest of the gospel.
Firm faith in resurrection turns negative doubt in v.2 ("Otherwise, you have believed in vain") into positive affirmation in v.58 ("you know that your labour in the Lord is not in vain").