John 3:3 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.
4 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?
5 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.
6 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
7 Marvel not that I said unto thee, Ye must be born again.
8 The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.
Being Born again according to Jesus is being born in the spiritual realm. It has nothing to do with our physcial bodies or our fleshly carnal minds. Because of sin our spirits are "dead". When we come to a place of belief and faith in Jesus' work on the cross our spirits are made alive. Our flesh and our carnal mind is not changed when we are born again. Only our spirit is revived and "born again".
God foretold of this in Ezekiel 36:26 A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh
If we believe the words of Paul on this subject our spirit is actually made alive in Christ and it is the spirit of Christ himself that begins to dwell in us when we are born again. Once this happens we engage in a battle between the sprit and the flesh (or our carnal mind, that is at emnity with God)
Romans 8
5 For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit.
6 For to be carnally minded is death; but to be spiritually minded is life and peace.
7 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be.
8 So then they that are in the flesh cannot please God.
9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
10 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.
Can one be born again but still live according to the flesh? Can you have the Spirit of Christ inside of you and still sin? That is the question that will forever be argued and debated. It's a question that is difficult for those who claim works are not relevant for salvation.
When we are born again, God looks at us and sees his Son in us. We oursleves become the sons of God.
John 3:3 Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
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.
11 For this is the message that ye heard from the beginning, that we should love one another.
According to John not too many people are truly Born again, because there are not too many people who do not commit sin. Did John really expect that we could live sinless after being Born Again? Notice that at the end of this passage John equates this idea of righteousness with the message of that we should love one another. Jesus himself summed up the law as "Love God and Love your neighbor as yourself" So the easiest way to define sin is through love. If anything is done out of a spirit other than love, it is sin.
1 John 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.
8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
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.
So obviously there is a dynamic here. When we are born again we have access to the ability to walk in the spirit of Christ. If we walk in his light we will not sin. But when we do sin, we confess those sins and he cleanses us form it. Cleansing is more than just forgiveness. When God cleanses he expects us to not sin again. But if we do we have an advocate.
1 John 2:1
2 My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:
2 And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world
So John's expectation is that we do not sin. However IF (notice he did not say when) we do sin, Jesus is our advocate before God.
Phillipians 2:12Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
Clearly Paul saw salvation as a process. The goal as a born again Christian should be to never sin. However it is not something that happens instantaneously when we are born again.
24 Then said Jesus unto his disciples, If any man will come after me, let him deny himself, and take up his cross, and follow me.
25 For whosoever will save his life shall lose it: and whosoever will lose his life for my sake shall find it
Only by denying the flesh (or carnal mind) and allowing the Spirit of Christ to reign in us, can we ever approach the goal of living without sin. The process of denying the flesh and walking in the Spirit is the process of Salvation.