Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God. Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born? Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. Marvel not that I said unto thee, Ye must be born again. John 3:3-7
The means of salvation is by being "born again." This is known as regeneration. It is the work of the Holy Spirit.
He saved us, not on the basis of deeds which we have done in righteousness, but according to His mercy, by the washing of regeneration and renewing by the Holy Spirit. Titus 3:5
The process of regeneration, or "new birth," causes the spirit of man and the Spirit of God to be joined together. Because this joining is with God the Holy Spirit Himself, this resulting ONE nature is everlasting. The spirit of man and the Spirit of God are forever "one spirit."
But he that is joined unto the Lord is one spirit.
1 Corinthians 6:17
Here lies the very heart of salvation. All of our understanding is built upon it.
From the time the Holy Spirit enters and regenerates a person forward they become ONE Spirit. The Bible no longer distinguishes between the sprit of man and the Holy Spirit within him. That is not to say that man is in any way God, or that the Holy Spirit is in any way contained within man's body.
Once joined, there is never again the possibility that the person may be separated from God.
For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord. Romans 8:38,39
It is through this oneness with God that the life in Christ is experienced. It is through this oneness that the Christian obtains the absolute assurance of his salvation and eternal destiny.
No rules of religion will ever be able to accomplish this joining together or "regeneration."
No rules of religion will ever be able to cut off man's spirit from God. No sins committed by the Christian in his body or mind can cause a split in the one spirit now at the root of his new nature.
When the person who has been born again dies his body returns to dust, but his spirit is freed to join with Christ where He is in heaven, seated at the right hand of God.