Mind and Human Nature

Catholics and most protestants almost always assume human nature is the state of man at birth, as in their concept of original sin where sin is passed to each individual at conception, but the New Testament uses "nature" in the sense of development after birth.

The way a person is trained in life affects their nature as an adult. A person's nature is what a person matures to become.

Consider

Gal 2:15 We who are Jews by nature, and not sinners of the Gentiles,

Notice the Jews by nature were in a sense more righteous and less sinful than their gentile counterparts. Yes Jews did sin but were less sinful than irreligious gentiles. Jews who had a religious upbringing were of a different mindset toward sin, at least in Paul's view.  Nature is used by Paul to distinguish between the two groups. If original sin were in view they would be sinners equally.

So the use of the term 'by nature" is in reference to human action, not just birth. This is also apparent in Romans 2:

Rom 2:14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:

Human nature is as much what man thinks as is what he feels. What we think is from teaching and training.

Eph 2:3 Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others.

We became the children of wrath because of the way we thought later than birth, as thinkers. We also obtain the heavenly nature by changing our desires. The word desire can be used of good or bad desires. By changing our desires we change our nature.

2Pe 1:4 Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.