Thank you for the theological question, “If I am in sin then what part of me is not in sin?”
There are two theological position on the nature of humans. The Arian view teaches that there is a divine spark within each person and the Augustine view is that every person is total deprave. Of course there are other views as man is not sinful but an evolved mammal without an spirit or spirituality.
As it relates to the question being asked, “If I am in sin, then what part of me is not in sin?” relates to the ability or inability of humans to redeem or not to redeem themselves before God.
From a Scriptural point of view, if a person is IN sin, then that person is in bondage to sin. That person’s mind, emotion, will, and even spirit is captivated to sin. There is an invasion into the body of a person, if I may use that terminology. That person is unable to break free from that bondage although he may break a particular habit of a sinful behavior.
Ephesians 2 says this, “As for you, you were dead in your transgressions and sins, 2 in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient. 3 All of us also lived among them at one time, gratifying the cravings of our flesh[a] and following its desires and thoughts. Like the rest, we were by nature deserving of wrath” (A searchable online Bible in over 150 versions and 50 languages). Bible citations are from the New International Version.
Romans 1 says this, “For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened…24 Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another…Furthermore, just as they did not think it worthwhile to retain the knowledge of God, so God gave them over to a depraved mind, so that they do what ought not to be done…(NIV).
John 8 says this, “Jesus replied, “Very truly I tell you, everyone who sins is a slave to sin.”
There are other verses and the ones indicate that one who is in sin is captivated to that sin and is unable to break free from the bondage of sin. There is not one part of that person that is free since the person’s mind, will, and emotions thinks of oneself and one’s self-centered life. Only a new creation, a new birth can change that person’s mind, will and emotion and given a new spirit from God.
SUMMARY: Every person sins against God and others whether one believes s/he has a sin nature or not. There are no sinless human beings except one, Jesus Christ.
For more perspectives:
https://www.quora.com/If-I-am-in-sin-then-what-part-of-me-is-not-in-sin