8 He who sins is of the devil, for the devil has sinned from the beginning.
For this purpose the Son of God was manifested, that He might destroy the works of the devil.
9 Whoever has been born of God does not sin, for His seed remains in him; and he cannot sin, because he has been born of God.
The answer is in the phrase
For this purpose the Son of God was manifested, that He might destroy the works of the devil.
and
'born of God', means God's seed remains in the person and they are saved for all time.
Jesus destroys the work of the accuser, the devil, and that which is born of God can not sin. Very true, can not sin is true.
Key to understanding this is for those born of God have a new spirit. That spirit is a new creation in Christ, that new spirit has eternal life and is joined to the Lord as one spirit with him, and that spirit can not sin.
But even though we have that new man inside, the old man of the flesh of us can and does sin.
So you have to understand, when we are saved, we are forever perfected spiritually, and are joined to the Lord as ONE SPIRIT WITH HIM.
That bond will never be broken.
You are the new man of the spirit, your flesh though is the old man and is dead due to sin.
Do not get caught up in the argument of losing salvation due to sin as it cannot happen.
People who lose salvation and depart from Christ, God had never granted them to come to Christ, and were never born of God. But you might have assumed they were, but it was fake news.
John 6
63 It is the Spirit who gives life; the flesh profits nothing. The words that I speak to you are spirit, and
they are life. 64 But there are some of you who do not believe.” For Jesus knew from the beginning who they were who did not believe, and who would betray Him. 65
And He said, “Therefore I have said to you that no one can come to Me unless it has been granted to him by My Father.”
Hebrews 12
The Glorious Company
18 For you have not come [
g]to the mountain that may be touched and that burned with fire, and to blackness and [
h]darkness and tempest, 19 and the sound of a trumpet and the voice of words, so that those who heard
it begged that the word should not be spoken to them anymore. 20 (For they could not endure what was commanded: “And if so much as a beast touches the mountain, it shall be stoned [
i]or shot with an arrow.” 21 And so terrifying was the sight
that Moses said, “I am exceedingly afraid and trembling.”)
22 But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, to an innumerable company of angels, 23
to the [j]general assembly and church of the firstborn who are registered in heaven, to God the Judge of all, to the spirits of just men made perfect, 24 to Jesus the Mediator of the new covenant, and to the blood of sprinkling that speaks better things than that of Abel.
1 Corinthians 6, One spirit with God, but not one flesh with God....
15 Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make
them members of a harlot? Certainly not! 16 Or do you not know that he who is joined to a harlot is one body
with her? For “the two,” He says, “shall become one flesh.” 17
But he who is joined to the Lord is one spirit with Him.
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John 6
35 And Jesus said to them, “I am the bread of life. He who comes to Me shall never hunger, and he who believes in Me shall never thirst.
36 But I said to you that you have seen Me and yet do not believe.
37 All that the Father gives Me will come to Me, and the one who comes to Me I will [f]by no means cast out.
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v37 also proves this. First you must be given by the Father to Christ, only then will you come to Christ and be saved, and He will never cast you away.