We are told we sin because, When Adam sinned his nature changed, so therefore everyone born from Adam's line has a new and unimproved sinful nature. What actually did change and when?
When we go all the way back to Genesis and see how it played out we can get to the root of the problem.
Eve ate of the forbidden fruit, she gave to Adam to eat as well, then their eyes were opened and they saw they were naked. Getting to the point here. God said, they have become like us, knowing good and evil. There you have it, a new nature. The only problem is, To say we have a sinful nature because we now know good and evil is to say the unthinkable, God has a sinful nature to. That doesn't even settle good with me to think it much less type it out.
So why do we sin? God created man to be in complete fellowship with Him from day one. Being estranged from God at birth and outside of that fellowship we are left to live by our own natural senses and abilities. Our flesh becomes our main focus. Our focus is feeding, clothing, making the flesh comfortable, etc... Our spirits are out of proportion to this world without God being in the driver seat.
The cure for sin.
The gospel gets preached, we hear the message and hopefully get saved. Fellowship is restored, the Holy Spirit moves in and helps us overcome temptation. It's all about fellowship with God.
Now we are told we have a new and improved divine nature. Not so fast there either. It is true we are new creatures in Christ, but we do not get a divine nature at being born again. The Holy Spirit moves in and fellowships with us, comforts us, and helps us overcome. Because of that truth it changes our perspective on everything and may feel as we have a new nature.
2 Peter 1:4 tells us we are now partakers of the divine nature and this is where we are told we now have a divine nature. When this is examined we see we are only partakes of the divine nature not that we get a new nature. The Greek word used here for partakers is also translated as fellowship and communion. So we see we are in fellowship with, in communion with, partakers with the divine nature.
If one were to give a description of God how would you go about it? What words would one used to define Him? Holy is one, but another is divine. So we see we do not get a new nature versus a sinful nature, but that now we are in fellowship with God, just as He planned all along.
Paul actually uses all three words, fellowship, partakers, communion, in 2 Corinthians 10:16-30. See also 2 Corinthians 6:14.
2 Corinthians 13:14; The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen. (communion here is the same Greek word used as partakers)
Hebrews 12:10; For they verily for a few days chastened us after their own pleasure; but he for our profit, that we might be partakers of his holiness.
Ephesians 3:9; And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:
Ephesians 3:16; That he would grant you, according to the riches of his glory, to be strengthened by his might by his Spirit in the inner man;
See also Ezekiel 36:26 and 27. It is the spirit of God that causes us to walk in holiness.
Revelation 3:20, 21; Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.
To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne.
See 1 John 2:27, 28 we abide in him.
It's not that we have a sinful nature or that we are broken when we are born that causes us to sin, but rather we are not complete in the sense that something is missing, the fellowship of God.
To have a visual of what is being said here just think of Moses coming down from Mt. Siani. They had to put a vail on his face because of the glory that was on it. Moses was in fellowship with God, now that is fellowship! Was Moses' nature changed? of course not, he even disobeyed God after that encounter with God on the Mt.