Where does this sin come from if not from the old sinful nature you got at birth from your ultimate earthly father Adam?
Sin comes from the devil. And it lives in the flesh. When I was in Adam I was a slave to sin, I was born into sin. But he who is born of God is NOT born into sin. I have no part of Adam's nature now - I am like Adam was BEFORE he fell. Just like Adam, I too can choose to eat of the wrong tree, I too can choose to listen to satan. But I'm not under the curse that Adam brought upon us.
"but we can choose to follow sin if we want."
If you had only one nature, the new one in Christ, there is no choice because your new nature is not influenced by sin in any way. Your new nature is the power you have in Christ over sin.
My new man, my inner man, my spirit, cannot sin. It is born of God. Rather than merely giving me power over sin, He let me DIE.
"When we were sinners by nature, we didn't really have a choice."
Right you were a slave to your sinful nature, which does not depart from the flesh when you are born-again. When you are born-again you receive power to overcome sin in Christ, that's not the same as removing the old nature.
Sin doesn't depart from the flesh, that's correct.
"The prodigal son's nature didn't dictate his behaviour."
Yes it did. Where did his desire to live for himself come from? Where did his desire to violate the commandment to honor his mother and father come from? His sinful nature.
But that wasn't his
nature - his
nature was that of being his father's son. He made some bad choices, but his nature and identity was always (and could never become anything other than) a SON. And it was because of his identity that he was immediately welcomed back.
"He thought he was an unworthy low-life, and therefore he acted like one."
Wrong, he came to that conclusion after he had demanded his inheritience from his father, squandered it all and was living in a pig-sty.
Yes, and it was a wrong conclusion. He wasn't unworthy at all.
Ha! I know for a fact that no part of you delights in God's law, as you have denied you are under God's law repeatedly. Every time God's law is applied to you in any way you say you are dead to it.
I have said several times that I agree with Paul that the law is good. That's not the same as being under it though. Swiss laws are brilliant, but that doesn't mean I am under them or should be under them or want to be under them.
The letter is good, but the Spirit is better.
It means the new nature you received wants to do good, but we all know that we do not always do the good we want to do, but instead do the bad that we don't want to do. Why is that? If the old nature were completely removed, this could not be possible.
As Paul says, he finds a "law" in his flesh, that goes against the "law" in his inner man. Paul himself wanted to do GOOD - that was his NATURE. But sin still hung on to his flesh. And that's why he talks so much about being
dead - "reckon yourselves dead to sin" and so forth.
"But that's not what the bible says. The bible says the old man DIED."
Okay, holo, show me your old man's body. Can't have a death without a body can you? Where is it? Where is your old man buried?
I have no idea where my old man is buried. All I know is that he was crucified and is dead. We died
spiritually, not physically.
Peter doesn't apply here because Peter had not been regenerated by the Holy Spirit yet. That was still to come.
True, and if Peter could defy the law of gravity even then, under the old covenant, before anybody had been born again and baptised in the Holy Spirit, then surely I can defy the law of sin and death too. By faith, by looking at Christ, just like Peter did.
If an unregenerate man like Peter could act as if gravity didn't exist - and it WORKED - then I'm definately going to act as if I'm dead to sin too. And that works too!

It's not just a theory, it actually works. It has worked against drug addiction, anger, porn addiction, the urge to tell lies, the tendency to be lazy, it has worked against belittling people who aren't around, etc etc etc, and it's still working, every day, all year.
It's all about walking in faith, fixing our eyes on Christ, and nothing is impossible!
"Actually, I have NOTHING from Adam. I have EVERYTHING from Christ

"
So when you still commit sin to this day, that sin is coming from Christ who never sinined? That doesn't make sense.
I can only say like Paul, that "so it is no longer I who sin, but sin that dwells in me..." sin is as serious as it ever was, but I will not be told that I WANT to sin or DELIGHT in sin. Just because I still somtimes forget who I am and act like the miserable sinner I used to be, doesn't mean sin originates in me or is something I take pleasure in.
People tend to confuse the fact that some sins give them a short-lived sense of fun or well-being, with an actual
desire to sin. Men who struggle with porn, for example; they may feel some sort of stimulation when they watch it, but it's not something they
want to do.