Jesus takes away all of our sin, including the sin nature; otherwise we couldn't partake of the divine nature.
We are able to partake now of what we have in part, but at the Day of Christ will with absolute certainty have in full. We are not fully rid of the "old man," as Paul calls it and you pointed out. This is why we are exhorted to
"put off (the) old self, which belongs to (our) former manner of life and is corrupt through deceitful desires, and... be renewed in the spirit of (our) minds, and... put on the new self, created after the likeness of God in true righteousness and holiness" (Paul, Ephesians 4:22-24), and to "lay aside every weight, and sin which clings so closely, and... run with endurance the race that is set before us, looking to Jesus, the founder and perfecter of our faith, Who for the joy that was set before Him endured the cross, despising the shame, and is seated at the right hand of the throne of God" (possibly also Paul, Hebrews 12:1-2). Peter exhorts us to the same thing in 2 Peter 1, as you pointed out.
Your question is then why do Christians sin? They don't know their own responsibility, and for some reason have taken into themselves a very apathetic view of sin from very bad teachers who will receive a stricter judgment...
Some underestimate the gravity of sin, certainly, and there can be apathy. But again, we have not completely escaped our old selves. Our sinful nature is still with us to some degree, and thus we still sin. This is what Paul even of himself ~ and thus is true of all of us ~ in Romans 7. But one great Day it will no longer be so.
Jesus was born with the same nature as we Christians have IF we have been born again of God.
Jesus
always had the very nature that we
will have when we are glorified. Remember the words of His prayer:
"And now, Father, glorify Me in Your own presence with the glory that I had with You before the world existed" (John 17:5). But He took on the same nature that we have had from birth at His birth ~ as Paul says, "
Christ Jesus... though He was in the form of God, did not count equality with God a thing to be grasped, but emptied Himself, by taking the form of a servant, being born in the likeness of men (and was)
in human form" (Philippians 2:5-8)
You still don't understand that all Mary did was give him a human nature...
Oh, I well understand that... :) But you don't seem to understand the what He had even long before (and long after) that... :) See above.
...not all Christians are born again.
If they are not born again, they're... not Christians. :) They might be at some point, though, if they are born again... :)
Those who struggle with sin have not been born again.
<chuckles...> See above. When one is born again and becomes a Christian, that is actually when the struggle against sin begins. :)
...Caused the human nature from Mary to be as Adam's was upon creation before he sinned and it became corrupt.
Hm. Again (from above):
Jesus
always had the very nature that we
will have when we are glorified. Remember the words of His prayer:
"And now, Father, glorify Me in Your own presence with the glory that I had with You before the world existed" (John 17:5). But He took on the same nature that we have had from birth at His birth ~ as Paul says, "
Christ Jesus... though He was in the form of God, did not count equality with God a thing to be grasped, but emptied Himself, by taking the form of a servant, being born in the likeness of men (and was)
in human form" (Philippians 2:5-8)See above.
This gave Him free will, and does the same for us. That free will is why we need to "keep ourselves" and to "purify ourselves."
1stCenturyLady, we all have free will, for sure. Nobody denies that. But our wills really are beside the point of anything we're discussing here, except that we have a responsibility to avoid sin and to glorify God, and that we should, as Paul says in Philippians 4, think about whatever is true, honorable, just, pure, lovely, commendable, excellent, and worthy of praise, and that we should practice what we have learned and received and heard and seen in him (because it is all really from God, of course). If we do, then the God of peace will be with us.
...our becoming holy is nothing we have done; it is all Christ. He made us slaves of righteousness. Romans 6:6-7; Romans 6:15-16.
Right. Agreed. Now, look again at what you just said here, particularly the word "becoming." It is not yet "became." :) But it will be. :) And we can and should live as if it is now. :; THIS is how we are partakers, even in this life.
Grace and peace to you.