The Bible uses several terms synonymously and equivalently to refer to the evil part of human nature.
God has revealed that humans are ruled by sin. This evil part of us hates God, desires wrong, cannot be tamed, is deceptive, is under the judgment of God and will not enter into heaven. The terms used equivalently are: sin nature, lower nature, old man, old nature, natural man, carnal man and flesh. When the term “flesh” is used in this spiritual sense it should not be confused with merely having a body. Jesus had a body, but he did not sin.
The sin nature (or any of the equivalent terms listed above) is the part of us that is inclined to sin. It is contrasted with the part of us that loves God and hates sin. It is natural for humans to sin. In fact, when people talk about “human nature” that is usually what they are referring to. When a person wants to justify their wrongdoing they will often state that “we are all human” in order to show how natural choosing sin really is to all of us.
In all cases the important concept is this: we have within a part of us that is evil. Prior to conversion and the new birth this evil part is the predominant part. Thus we live in an evil world where humans respond to life wrongly and sinfully because of an inner inclination to sin. This world and the humans in it are therefore at odds with God. Now, even after a person partakes of forgiveness through the propitiation of Christ and even after such a person is reconciled to God and is given a new nature, that person still has a sin nature. We still have the “flesh” to deal with. This is why we are told to “crucify the flesh”, to “walk in the spirit and not fulfill the lusts of the flesh” and to “put off the old man”.
Christians have a dual nature. Two parts, one from below and one from above. They are in conflict with each other and victory over the sin nature becomes a major theme in the New Testament. This victory includes concepts such as temptation, contrition, repentance, mortification, abiding, reckoning, sanctification and growth through suffering.