The nature or constitution of man consist of two different classes of humanity (1) one being, those who are 'in Adam' as unbelievers from birth and the other; (2) those who are 'in Christ' as born again believers deriving from a supernatural rebirth. Scripture also describes those who are 'in Adam' as the 'old man', possessing an inherit nature and sharing the sinful characteristics of Adam from a fallen state (genesis 5:3). In a similar fashion, Scripture also describes those who are 'in Christ' as having obtained a new nature possessing the righteous characteristics of the 'new man' which is found in Christ.
However, there remains an obvious and glaring DIVIDING LINE between the 'old man' and the 'new man' - this dividing line is the Holy Spirit. It is the Holy Spirit either IN MAN or NOT that is the dividing line between the two natures of man; that being IN CHRIST having the Spirit or IN ADAM without the Spirit.
Since the Holy Spirit is the "dividing line" between the two nature's of mankind, let's now look at a few characteristics of both.
For example:
The 'old man', because of his nature can only do what his nature allows him to do; he is indeed confined. The old man or unbeliever can only obey sin and not righteousness (romans 6:12-14, 16-19). Furthermore, the old man only obeys the cravings of the flesh and following it's desires and thoughts because of the power and control of sin in his body (ephesians 2:3). Sin and it's control and power is the 'old mans' Master (romans 6:14). Sin REIGNS in the 'old mans' mortal body therefore making the unbeliever obey it's passions (romans 6:12). The flesh alone serves only the Law of sin (romans 7:25). Man born of the flesh is born sinful by his very nature. We are born with a SINFUL NATURE being that of the 'old man'. And by our very nature from birth, we are considered children/sons of disobedience deserving wrath (ephesians 2:3, galatians 2:15) - and just why is that? Because we are born with a nature that can ONLY and WILL inevitably sin leading to death apart from the initial unilateral saving work and grace of God.
However, in regards to the New Man and his nature, he is not confined to only obey sin and what the flesh desires but he has been freed from the power and enslavement of sin and what the flesh desires and therefore can now freely obey righteousness by the Spirit of God in him.
For example:
The 'new man' because of his new nature is not only free to obey sin but now righteousness as well, a privilege the 'old man' does not share. Wherever the Spirit of the Lord is, there is freedom (2 corinthians 3:17). The 'new man' according to his nature is comprised of those who are to be found 'In Christ'. They are those who have been born again by the Spirit of God. These born again believers or the 'new man' serve or obey God in a new way according to the Spirit and not by the Letter of the Law that is according to the flesh (romans 7:6). Having become slaves of righteousness and set free from sin, the 'new man' has now become obedient from the heart whereas the 'old man' was by nature stubborn and disobedient from the heart (romans 2:5), seeing that he was enslaved to sin according to his nature and can only obey unrighteousness (romans 6:17-19). Lastly, the 'new man' with a new nature who is now enslaved to righteousness having become born again has the power and ability to mortify the deeds of the flesh BY THE SPIRIT; another privilege the "old man" doesn't possess because of his nature being void of the Spirit; this being a distinct difference between the two nature's. The 'new man' shares in Christ Divine nature (2 Peter 1:4) and obviously, the 'old man' does not.
Point being, without sounding redundant - the 'old man' can only obey sin and the 'new man' has been freed from only obeying sin but now righteousness as well. This fact, is due to the difference in the 'old and new mans' nature. The 'old man' can ONLY bear fruit unto death, the 'new man' bears fruit unto God; both bears fruit, be it ACCORDING to their NATURE.
By Aaron Fisher.