The idea of the dual nature was condemned in the first century as the Gnostic heresy.
To say that sin is inevitable is to attribute to Satan power he no longer has. The Bible teaches that the old man is dead.
Rom. 6:6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
Col. 3:9 Lie not one to another, seeing that ye have put off the old man with his deeds;
2 Cor. 5:17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.18 And all things are of God, who hath reconciled us to himself by Jesus Christ, and hath given to us the ministry of reconciliation;
If you are in Christ, the old man has passed away, and all things are new, and ALL THINGS ARE OF GOD. Temptation however does not cease, but even with the temptation we are given assurances by God that He will not allow us to be tempted beyond our ability to withstand.
1Co 10:13 There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.
The enemy might lie to you and convince you that you cannot be free of his control. You have been taken captive to continue to obey his will.
2Ti 2:24 And the servant of the Lord must not strive; but be gentle unto all men, apt to teach, patient,25 In meekness instructing those that oppose themselves; if God peradventure will give them repentance to the acknowledging of the truth; 26 And that they may recover themselves out of the snare of the devil, who are taken captive by him at his will.
The gnostics believed the flesh was inheritably evil, but the spirit was inheritably good. They taught that regardless what you did in the flesh, your spirit was not affected. They were going to whores, and getting drunken and claiming to be in good with God. To them their spirits were good and could not be corrupted by the flesh. That is what John was talking about in 1 John 2:19. People who sin and then justify it. Notice there are 4 places where John referred to lying or liars:
1) 1Jo 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.6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:
2) 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.8 ¶ If we say that we have no sin, we deceive ourselves, and the truth is not in us.
3) 9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. 10 If we say that we have not sinned, we make him a liar, and his word is not in us.
4) He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
Here is the Gnostic heresy. They claim to walk with God but continue to walk in darkness by sinning. But then they claim to have no sin because of the dual nature. But by saying they have not sinned they make God out to be a liar. Why? Because he that says he knows God but does not keep His commandments is a liar.
1Jo 3:3 And every man that hath this hope in him purifieth himself, even as he is pure. 4 ¶ Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law. 5 And ye know that he was manifested to take away our sins; and in him is no sin. 6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him. 7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous. 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. 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. 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.
This clearly says that anyone who is born of God does not sin any more. Anyone who sins is of the devil, and not of God. And it is not because of a dual nature. Verse 7 equates the righteousness of a believer to the Lord’s righteousness. “he that doeth righteousness is righteous, even as he is righteous”. The second he in that verse refers back to the Him in verses 5 and 6, which is Jesus. Did Jesus have an old man corrupting Him? As to the absolute necessity to cease from sin, the Bible says this:
Heb 12:14 Follow peace with all men, and holiness, without which no man shall see the Lord
As to the ability to do this in this lifetime, (cease from sin), we have this passage in 1 Peter.
1Pe 4:1 ¶ Forasmuch then as Christ hath suffered for us in the flesh, arm yourselves likewise with the same mind: for he that hath suffered in the flesh hath ceased from sin; 2 That he no longer should live the rest of his time in the flesh to the lusts of men, but to the will of God. 3 For the time past of our life may suffice us to have wrought the will of the Gentiles, when we walked in lasciviousness, lusts, excess of wine, revellings, banquetings, and abominable idolatries:
This says ceasing from sin happens in this life before death. Death is nothing more than a cop out for most people who refuse to take on the suffering necessary to cease from sin. Death while you are still in your sins will not suddenly fix you. It will send you to hell.
//Thanks to BRO COPE