Paul continues to contrast the differences and the similarities between the natural body and the spiritual body (1 Corinthians 15:44) because this contrast is the main point of this letter to the Corinthians. He has been talking about nothing else from the very beginning of this letter (1 Corinthians 2:12-14).
In verse 45 Paul contrasts several elements of the spiritual body with the natural body, all of which are very important. First of all, to suggest that Jesus was like Adam is nothing less than astonishing. Because of the nature of inheritance Adam is the primary model for all human life. We are what we are because Adam was what he was. He was the original, we are the copies, and the absolute best that a copy can be cannot exceed the quality of the original.
Adam's creation predated the entry of sin into the world. Thus, Adam was created without sin. And the comparison tells us that Christ, who is a type of Adam was also without sin. Scripture goes on to tell us, however, that Adam did sin, and that he sinned prior to the birth of any children, and that Adam's sin changed the relationship between God and Adam that would effect all of his natural children (Genesis 3). As history cannot be erased, so Adam's sin was historical and the reality of that history necessarily continues through time and accrues to Adam's posterity and to us. Adam played a unique role in the history of humanity as the first of a kind.
Adam was unique, so Paul's reference to Jesus Christ as the "last Adam" (1 Corinthians 15:45) cannot be overstressed. Paul tells us here that Jesus Christ is the most important person in human history since Adam, since the very beginning. And He goes on to say that Christ's importance far exceeds Adam's. We need to pay close attention to this.
Paul also tells us that "Adam became a living being" and Christ "became a life-giving spirit" (1 Corinthians 15: 45). In Genesis 2:7 we learn that "the Lord God formed the man of dust from the ground and breathed into his nostrils the breath of life, and the man became a living creature." The life that God gave Adam has been passed down through the ages through what we call the natural process of generation or reproduction, and Paul refers to the product of natural birth as the "natural man" (1 Corinthians 2:14).
In contrast, Christ was miraculously born from a virgin, suggesting that His biological inheritance was more like Adam's in that both Adam and Christ were more directly related to God than the rest of humanity, whose relationship with God is more distant. Christ's nature was similar to Adam's nature in that both were immediately related to God. While the rest of humanity could claim that God Himself was their great, great, great (ad nauseum) grandfather, both Adam and Christ were immediate or direct sons of God.
And yet Paul also tells us that Jesus Christ "is the image of the invisible God, the firstborn of all creation" (Colossians 1:15). Adam preceded Christ in time, and yet Christ precedes Adam in eminence as the firstborn Son. Paul speaks repeatedly of Christ as the firstborn. Indeed, Adam and Adam's progeny have new life in Christ inasmuch as they are born again in Christ. In fact, the primary story of the Bible is the story of the Fall of humanity into death through Adam and the regeneration or resurrection of humanity into new life through Christ.
Christ is the firstborn of the reborn. In the natural person of Jesus is fused the supernatural Person of the Holy Spirit. When "Jesus also had been baptized and was praying, the heavens were opened, and the Holy Spirit descended on him in bodily form, like a dove; and a voice came from heaven, 'You are my beloved Son; with you I am well pleased'" (Luke 3:22). Thus, Paul's contrast between the first Adam and the last Adam is a contrast between the natural and the supernatural, between generation and regeneration, between the wisdom of the world and the wisdom of God (1 Corinthians 1:20-21). This contrast is the central point of Paul's first letter to the Corinthians.
The King James Bible contrasts a "living soul" with a "quickening spirit." Within this contrast is the difference between "living" and "life-giving" and between "being" (soul) and "spirit." Rather than getting distracted by the many subtleties of the Greek, let me just note that all of these contrasts point to the difference between natural and supernatural, between generation and regeneration, between the wisdom of the world and the wisdom of Christ.
Paul goes on in verse 1 Corinthians 15:46 to tell us that while Christ was first in eminence, Adam was first in history. It is critically important that Adam was first in history because historical movement from Adam to Christ is a function of regeneration in or through Christ, while historical movement from Christ to Adam would be a function of degeneration. It is significant that history moves toward Christ or into Christ, not away from Him. In Christ is the hope of regeneration, restoration and wholeness, and that hope is the engine of history. History is powered by hope in Jesus Christ. If history flowed in the other direction, from Christ to Adam, there would be no hope. History would flow from life in Christ to death in Adam. History would be degenerate rather than regenerate, and all humanity could only wallow in hopelessness. But because history flows from Adam to Christ, from death to resurrection, there is hope -- and more than hope, proclaimed Paul, resurrection in Christ is a certainty. First "the natural, and then the spiritual" (v. 46).