So there is a duality, the Flesh vs the Spirit.
The duality is indeed spirit and flesh, not spirit vs flesh: immortal spirit and mortal flesh is the duality of man on earth.
Our fight is now the our own bodies, but with the devil tempting us to receive lust within our hearts, that we soul then sin against God with out bodies.
(For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds;) Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ.
The flesh or the soul being made and born with sin nature is a lie that sinners and hypocrite christians use to excuse what they are and what they are doing:
"It's not my fault, God created me this way." or
"I was born this way."
The only difference between man and angels, is that we are immortal created beings in mortal bodies, while the angels have spiritual immortal bodies only, which is how we are therefore made a little lower than the angels wrapped in mortal flesh.
Natural man is not the body, nor is the hapless mortal body an old man of sin, pumping out lust through the body from a 'sinful' heart of flesh.
Out of the sinful heart of the soul proceeds all evil thoughts to do evil on earth, against God in the spirit first and then against man with the flesh.
Natural man is simply any man that rejects the things of the spirit and says there is no God nor soul: he gives himself entirely to the body and the things of this life. It does not necessarily make him evil, and he can be quite neighborly at times, but he cuts himself off from the Spirit and grace of God by unbelief.
If we are lusting for the flesh to sin with the flesh, the flesh has nothing to do nor say about it, being but mortal flesh, but rather our soul and spirit are receiving such lust within, and transgressing the first great commandment of the pure in heart: to love God with all the heart.
If you do not want to walk after the flesh, then you must work with the power of Christ given you, to cast down and out any unrighteous thoughts of the devil knocking at your doorstep, and put them in obedience to Christ. In this way you do not allow the devil and sin to have place within your heart, and so not to have sin reign over your mortal body:
And when the fowls came down upon the carcases, Abram drove them away.
Calling the mortal body the old man of sin born with sin nature, is the perfect lie of the devil for them that still enjoy sinning in the flesh: they think to separate the will of their soul from the deeds of their body.
With strong delusion they actually say, it's no more they which do the sinning, but only the flesh
without their spiritual consent.
They also say their old man of physical body is now dead, and yet their spirits remain therein.
They are literally teaching the scripturally psychotic notion of their physical bodies being beheaded, while spiritually holding to the Head Jesus Christ.
OSAS is christian zombie-land: their old dead corrupt bodies keep doing evil things, while claiming their souls are spiritually pure and preserved in heavenly places, like so much jelly and jam in glass jars.
They think they are like those eternally preserved minds in glass bubbles from Star Trek, just waiting to get their brand-spanking new spiritual bodies, while their old hapless 'sinful' body just keeps on doing what it wants to do on earth.
One thing is for sure: while they blame their bodies for sinning against God, they certainly like the credit for doing anything good.
While they no doubt say, don't thank me, thank Jesus in me, they will never say don't thank me, thank my body that just does what it wants to do, whether good or evil.