We are born sinless and innocent, but with a sin nature that tends toward sin at every opportunity. It is this sin nature that is dealt with at the cross, and are given a new born again nature as we accept Christ's resurrected life as our own.
Obviously we did not sin prior to conception. Most would not understand sin as a concept while sitting in the womb. But sinless means more than just sinning for the first time.
Why equate that with nature though? Paul says it is this body that wants to sin, not nature. So I am not sure how replacing a nature really explains things. We are to have the mind of Christ, and submit our will to that of the Holy Spirit.
Besides it is this body of death that cements us into death itself, not our mind. That we are born sinners should be understood within the first 5 chapters of Genesis.
"And Adam lived an hundred and thirty years, and begat a son in his own likeness, and after his image; and called his name Seth:"
In conjunction with Romans 5
"Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:"
So what is passed down from generation to generation? A physical body, death or death in that physical body?
At what point is one a sinner? At conception or when they come face to face with God?
To me, teaching children or adults they are sinless and innocent sends the wrong message to begin with, because then we have to convince them after years of thinking they are sinless and innocent, that they are now a sinner, by what they do, instead of who they are. All have sinned, and that is not predicated on a certain age.
It is possible that some never sin, because of some unfortunate physical condition of the brain that never allows full mental development. And hundreds of millions don't even make it to term. And many innocent children die in the millions each year. All are covered by the Cross, until the point they can face God and make that decision on their full God given abilities to do so.
I don't think Paul was saying that sin passed on from generation to generation, but death did through the physical body of death. Because this body is of death, all have sinned, even if they don't even recognize sin as sin. Many societies have tried to replace God with their own culture and traditions. I think the Holy Spirit is always at work though, but in some cases it may seem hardly an effort, being suppressed by culture. That is why the Gospel is to go out and bring light to the cultural darkness.
Saying the nature of man was placed on Christ at the Cross is misleading unless the point is that it is all sin that was actually placed on Christ. Some want to limit that sin to just those who are going to escape the LOF. What is the point of limiting sin to the end of time, when Revelation points out the Lamb was slain from the foundation of the world? There was no sin at that point.
2 Corinthians 5:21
"For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him."
Would any equate the Second Birth as making us sinless and innocent, until we mess up? Can we fall in and out of either birth?
Could the second birth be a process that we choose to start and is not complete until physical death? Not that we can be unborn, and loose our salvation. I don't see the reality of renewing this dead flesh ever, as it is the mind that changes, and not the physical body, until we leave this body behind. God is not in the process of changing this physical body. God is in the process of changing the soul to be conformed to the final image we obtain once we shed this image of death.
Paul puts it this way as it relates to the soul which is our mind and will.
Romans 7:14-25
"For we know that the law is spiritual: but I am carnal, sold under sin. For that which I do I allow not: for what I would, that do I not; but what I hate, that do I. If then I do that which I would not, I consent unto the law that it is good. Now then it is no more I that do it, but sin that dwelleth in me. For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not. For the good that I would I do not: but the evil which I would not, that I do. Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me. I find then a law, that, when I would do good, evil is present with me. For I delight in the law of God after the inward man: But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am! who shall deliver me from the body of this death? I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin."
This is the struggle between the soul that is of the Second Birth and the body of flesh. This is what is called the "two natures". But I am not seeing how the mind is instantly changed and becomes a different nature as many define it. The soul is redeemed no longer held accountable for the sin of the physical body of death, yes. But the struggle is that the soul is being conformed to the will of the Holy Spirit, not that the struggle is removed and the soul is now instantly changed into something new. It was the soul born into this body of death, that never wanted the desires of the flesh, but that is all the soul knew until another law or nature was applied to the soul pointing the soul into a different direction of mind and will. That is what the Holy Spirit is for since conception. But culture has dictated that the Holy Spirit is not there at conception, and the working of the Holy Spirit is limited until the light shines that it was the Holy Spirit all the time at work.
That is why I don't see it as automatically getting a "new nature", but having the ability to allow the Holy Spirit to actually work in us. I think it sets a wrong precedent to say the Holy Spirit only enters at a point of a second birth. Now one can argue that the Holy Spirit was limited in the OT, but I think that is wrong as well. The Holy Spirit was given to that group of chosen on a particular day, in a physical manifestation, which is true. That does not happen physically at every Second Birth. And those were already partakers of the Second Birth when Jesus chose them, not at the moment they received the physical manifestation on the day of Pentecost.
In that passage from Romans 7, Paul mentions several laws which we will assume the term "law" many refer to as "natures". We have the law/nature of the soul/mind. There is the law/nature of God, the Holy Spirit at work, and the law/nature of sin/the body of death. So the soul does not necessarily receive a different nature than it already has from conception. The point is made does the soul follow the will of the Holy Spirit or the will of sin and death, represented by the physical body.
One could then argue that sin is corporate of all Adamkind or mankind through the line of Seth. Individually does not effect even in the billions the entire death that was passed on because of one man's disobedience. Not that it changes the suffering of Christ, but that Christ was made sin for every single human, not just a select few, because all came through Adam and Eve via Seth and his offspring. All are born sinners because they are given the body of death in the womb, as well as the Holy Spirit at work to provide the means for the soul to receive redemption.