What part of you is in Christ? The inner man? Is your body in Christ? Old things about your inner man have passed away, and all things about your inner man are become new. Sadly, the inner man still dwells within a sinful body of flesh. Our flesh will be changed one day, but until then, it becomes a daily battle.
The "flesh" isn't sinful.
The word for "flesh" in the Greek is Sarx and all it means is the "meat on the bones." It is a reference to our natural physical body and thus also includes the natural passions and desires, neither of which are sinful.
The flesh is only sinful in the sense that human beings sin in the flesh. Thus when Paul used the term "likeness of sinful flesh" he was speaking in that sense.
The Gnostics opposed the idea that Jesus came in the flesh because such would put sin in Jesus. This is what John is correcting in writing...
1Jn 4:3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
The Gnostics would teach that the "manifest conduct" did not have to match the "confession of the mouth" because they viewed the flesh as sinful, hence their doctrine was a duel nature doctrine (two natures existing in one being).
John very clearly explained that...
1Jn 1:5 This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
1Jn 1:6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:
1Jn 1:7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
Light and darkness do not mix. The cleansing available via the blood is wrought through WALKING in the light, hence one must be yielded to God and WALKING according to the Spirit.
When one approaches God in "repentance and faith" (ie. honest,obedience and trusting heart) then a cleansing occurs (Heb 10:22). John writes...
1Jn 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
1Jn 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
1Jn 1:10 If we say that we have not sinned, we make him a liar, and his word is not in us.
John is speaking of the INITIAL CLEANSING al...
Heb 10:22 Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
Not some ongoing cleansing repeated over and over again.
Those who are genuinely in the faith WALK THE WALK as well as TALK THE TALK which is why John wrote...
1Jn 2:3 And hereby we do know that we know him, if we keep his commandments.
1Jn 2:4 He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
1Jn 2:5 But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.
1Jn 2:6 He that saith he abideth in him ought himself also so to walk, even as he walked.
The love of God is perfect in those whom WALK in the light as He is in the light, which is why the Saints are implored to "keep themselves in the love of God" (Jud 1:21) and that the "love of God" is "keeping His commandments" (1Jn 5:3).
The deception of false Christianity is like a whore whom leads one into a deep ditch and is able to catch the simple and thus increase their rebellion (Pro 23:27-28). In other words it presents a line of thinking in which one can "sin and not surely die" which is Satan's very first lie.
There are two kinds of people in the world. Two kinds!
The Wicked.
The Righteous.
The Wicked DO evil.
The Righteous DO righteousness.
1Jn 3:6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
1Jn 3:7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous.
1Jn 3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
1Jn 3:9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
1Jn 3:10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
1Jn 3:11 For this is the message that ye heard from the beginning, that we should love one another.
There is no such thing as "abiding in Jesus" and "sinning not" because of some duel nature where one is "pure in spirit" but "wicked in the flesh." That is nonsense which is why John used the term MANIFEST in 1Jn 3:10.
The children of God are MANIFEST to the world in that they DO righteousness.
The children of the devil are MANIFEST to the world in that they DO NOT righteousness.
As for a history of Dual Nature doctrine, I made a documentary about 5 years ago on the subject which is thoroughly researched and annotated. It is well worth examining...
Augustine and Original Sin
[video=youtube;KVQ1t5i058Q]https://www.youtube.com/watch?v=KVQ1t5i058Q[/video]