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, 1st John 3:9.
There is no single stronger assurance of eternal security in the word of God than 1st John 3:9. The Arminian or the person who believes that the new nature of a believer can die is forced to change that verse to keep his theology. Oddly, fundamentalists have long had a dispute with that verse. They try to change it to "does not practice sin" instead of cannot sin. That is not what the verse says.
The new nature in a believer is born as pure as the birth of Jesus Christ himself. It is a spotless birth in which the new nature born inside of the newly converted believer is as holy and pure as the baby that lay in a manger in Bethlehem. The Apostle Paul spoke of this when he said; And that ye put on the new man, which after God is created in righteousness and true holiness, Ephesians 4:24.
What could be more righteous and truly holy than to be incapable of sin by birth? Some may object that Paul spoke of a created nature whereas John spoke of a birthed nature. I ask the reader, were you born or created? The answer is that you are both. Every baby in a hospital nursery is a creation from God birthed to its mother. As usual, the Apostle Paul is more detailed. Under what precept would the Lord bestow such a great blessing on those who heard John or Peter preach and then deny the greatest single blessing ever given to man to those who were converted under the Apostle Paul?
The Apostle Paul spoke of living with such a contradiction in natures, a body of flesh prone to sin and an inward man incapable of sin. I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin, Romans 7:25. If we look into Paul's statement a little deeper we find that the good apostle is describing the inward battle of a person born of God.
For that which I do I allow not: for what I would, that do I not; but what I hate, that do I, Romans 7:15. What is a person in reality? Is he what we see, or is he what is on the inward part? For as he thinketh in his heart, so is he, Proverbs 23:7. Or, as Samuel said; for the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart, 1st Samuel 16:7. In Romans 7:15-25 the Apostle Paul expresses his anguish and his solution to having two distinct different natures operating within him.
He is telling us that inside of him (the real him) is a will to God's will. That is the polar opposite of an unconverted person who struggles with religious affections or sense of duty, but in his inward part desires sin. Watch the following verse and see that Paul identifies his true inner self to have a will for God. As he thinketh in his heart, so is he. What keeps religious people lost is their unwillingness to see their true heart and admit they they are at heart, breakers of the laws of God.
Romans 7:15 For that which I do I allow not: for what I would, that do I not; but what I hate, that do I.
Romans 7:16 If then I do that which I would not, I consent unto the law that it is good.
Romans 7:17 Now then it is no more I that do it, but sin that dwelleth in me.
Romans 7:18 For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
Romans 7:19 For the good that I would I do not: but the evil which I would not, that I do.
Romans 7:20 Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
Romans 7:21 I find then a law, that, when I would do good, evil is present with me.
Romans 7:22 For I delight in the law of God after the inward man:
Romans 7:23 But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.
Romans 7:24 O wretched man that I am! who shall deliver me from the body of this death?
Romans 7:25 I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin.
That is the testimony of the new birth. It is the testimony of a man whose heart is pure and holy because it is born of God. Because Paul doesn't call it that, the person who lacks understanding can't see it.