This is a difficult concept to put into words, so my upfront apologies in case I don’t do a good job with it.
I think the way I look at it is that our soul and body are both connected but separate, but I don’t know that I would consider our natures separate. I would say that we have one nature, but two things competing for that nature, the divine and our human flesh. Kind of like a tug of war. Before Christ, our nature is being pulled by our flesh with little resistance. (think Shaquille O’Neal vs. Urkle). But after Christ it is more like Shaq vs. the world’s strongest man. And the more we grow in Christ the more strong men we get on that side.
So I am not sure that I would say he is changing one nature into another, rather that he gives the devine the strength to withstand the human. The competition for our nature is always present as long as we are in these bodies but we get better at resisting the flesh the more we grow in Christ.
As far as why he chose to use human life as the vehicle for us, I think that it ultimately an unanswerable question. If he doesn’t tell us in His word, then all we can do is guess and speculate. Obviously there is a lot that he has chosen not to directly tell us which has led to a lot of guessing and speculation on our part. I think the short answer is that, God is all knowing, God is perfect, so the reason He chose this method is because it was the best way. He came up with the way that is the most fair, the most just, and ultimately gives us the ability to chose Him. I know it feels like kind of a cop out answer, but with any ultimately unanswerable question, I think that is how any answer feels.
@Susan_Baker Good question! I do not think that whether or not the soul is inherently immortal is a question that needs to be answered to understand what Jesus did for us. Though I did include a link below addressing that topic.
In answer to (1) Jesus gives us a new heart - this something that happens in the spiritual realm. The Holy Spirit brings our heart to life and indwells us. When the Bible talks about us having a divine nature, Peter defines that as escaping the corruption that is in the world because of evil desires. In other words, God sets us apart in Christ as holy for Him - not to be used for immoral or evil purposes - but set apart unto God. We also receive an imperishable body - a spiritual body - when we are resurrected - like that of Christ.
I Corinthians 15:42-44 - So will it be with the resurrection of the dead. The body that is sown is perishable, it is raised imperishable; 43 it is sown in dishonor, it is raised in glory; it is sown in weakness, it is raised in power; 44 it is sown a natural body, it is raised a spiritual body.
2 Peter 1:3-4 - His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, 4 by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire.
In response to (2), Jesus had to die in order to reconcile us - our whole being - to God.
It is generally accepted that we were created to be a body / soul combination - not one separated from the other. That is why in Romans 8 when Paul talks about the Spirit giving life he also talks about the redemption of the body (Romans 8:22-25). After Jesus returns we will be resurrected with a new body in a new creation - not disembodied spirits.
In Romans 8 and Galatians 5 we see that even if a human knows the law of God in their mind, they cannot keep it because of the evil desires of the flesh. When Jesus’ died on the cross and gave us the gift of His Spirit, He gave us power to walk in victory over the flesh and obey the law of God from our heart. He gave us a new nature - a new heart of flesh rather than a heart of stone (Ezekiel 36:26). We now obey the law of the Spirit of life rather than the law of sin and death (Romans 8).
2 Cor 3:3 - You show that you are a letter from Christ, the result of our ministry, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.
2 Cor 3:18 - And we all, who with unveiled faces contemplate the Lord’s glory, are being transformed into his image with ever-increasing glory, which comes from the Lord, who is the Spirit.
I John 3:2 - Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when Christ appears, we shall be like him, for we shall see him as he is.
When we die, we will no longer have to live in a body where we still have to wage war against the desires of our flesh. Jesus has set us free from sin’s power and from sin’s penalty, but when we go to be with Jesus we will finally be free from sin’s presence.
Summary of the topic from the ESV Study Bible:
Biblically, there are at least two distinct aspects of a human being—spiritual (spirit/soul) and physical (body). Some interpreters hold that the “soul” and “spirit” are distinct parts of a human being, and therefore that we are composed of three parts: body, soul, and spirit. This view is called “trichotomy.” However, the vast majority of evangelical scholars today hold that “spirit” and “soul” are basically synonymous and are two different ways of talking about the immaterial aspect of our being, “soul” pointing to our personal selves as responsible individuals and “spirit” pointing to those same selves as created by and dependent on God. This view is called “dichotomy” (see note on 1 Thess. 5:23–28). It is important to see that there is a fundamental unity between the physical and spiritual within humans. While a distinction is made in the Bible between the material and immaterial parts of the human being, the emphasis is on the necessary connection between body and soul. Regeneration and sanctification for the Christian is a spiritual experience intended to be expressed in the physical body in and through which we have been made to live. The separation of body and soul caused at death is an unnatural tragedy, which will be remedied when the body is resurrected, allowing humans to exist as they were intended to do.
Thank you. I really appreciate your careful reflection. I asked this for an atheist friend and because, I too, am challenged to understand how we will be able to be in God’s Kingdom as human beings. I’ve learned the verses that address this, but I repeat the words without understanding how. It doesn’t worry me to question this. I don’t want to diminish God’s work by not understanding and 1Peter 3:15 calls me to have an answer. Your reply will help me continue to pray and seek His truth. God bless!