The Bible says all of us have a sin nature. After Adam and Eve sinned, they began to have sons and daughters in their own likeness.
When Adam had lived 130 years, he had a son in his own likeness, in his own image; and he named him Seth (Genesis 5:3).
Adam's likeness means with a sin nature. All of us born since have that sin nature. This nature is characterized by a tendency to serve self rather than God. Its tendency is to leave God out of our life and actions. The old sin nature is constantly urging us to sin. The old sin nature wants to do evil rather than good or to do neutral things that exclude God from the picture.
Problem Stated
The constant struggle with our old sin nature is pointed out in Scripture.
Those who are unspiritual do not receive the gifts of God's Spirit, for they are foolishness to them, and they are unable to understand them because they are spiritually discerned (1 Corinthians 2:14).
Those who have not become Christians only have the capacity to serve self. The old nature is always bad and it cannot be improved.
After Salvation
Prior to salvation we could only serve our selves. This is the only capacity the non-Christian have. Once a person trusts Christ they now have a new nature. However the old nature does not go away. It will not leave the believer until the Christian goes to heaven. We are told not to encourage the old nature in any way.
Rather, clothe yourselves with the Lord Jesus Christ, and do not think about how to gratify the desires of the sinful nature (Romans 13:14).
New Nature
The Bible says that all believers receive a new nature upon their trusting Christ as Savior. The new nature we have cannot sin.
Mind Of Christ
The Bible says that we also have the mind of Christ:
For who has known the mind of the Lord that they may instruct Him? But we have the mind of Christ (1 Corinthians 2:16).
Constant Struggle
Prior to salvation the non-Christian has only one capacity, to serve self. At the moment of salvation the person receives a new nature where they have the capacity to serve God. There is now the tendency to do what God wants them to do. They have the capacity, at any given moment, to serve God or to serve self. There is a constant struggle between the old and new natures. Scripture says.
Let us not become conceited, provoking and envying each other. For the sinful nature desires what is contrary to the Spirit, and the Spirit what is contrary to the sinful nature. They are in conflict with each other, so that you do not do what you want (Galatians 5:16,17)
Those who live according to the sinful nature have their minds set on what that nature desires; but those who live in accordance with the Spirit have their minds set on what the Spirit desires. The mind of sinful humanity is death, but the mind controlled by the Spirit is life and peace; the sinful mind is hostile to God. It does not submit to God's law, nor can it do so.
Life-Long Struggle
The struggle between the two natures will continue during the entire life of the believer. Deliverance is by means of the Holy Spirit working through new nature to hold down strong desires of old sin nature.
Summary
A non-Christian operates entirely on human wisdom-they are unable to understand spiritual things. After one becomes a Christian, there is now a choice. The old sin nature remains and a constant struggle is going on between the two.