we all have one nature before being born-again, a sin nature. Once we are born-again we then have two natures that oppose each other, The Spirit and the flesh. To think that the flesh will NEVER have a victory after we take on the new nature is just not reality. We are born in Christ as babes, not as adults (spiritually speaking). Do we expect babies to act as adults or do we extend them the grace to grow into adulthood?
This stuff isn't brain science, it's rocket surgery!