1Jo 3:1 ¶ 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.
 12 Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.
 13 Marvel not, my brethren, if the world hate you.
 14 ¶ We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death.
 15 Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.
 16 Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren.
 17 But whoso hath this world's good, and seeth his brother have need, and shutteth up his bowels of compassion from him, how dwelleth the love of God in him?
 18 My little children, let us not love in word, neither in tongue; but in deed and in truth.
 19 And hereby we know that we are of the truth, and shall assure our hearts before him.
 20 ¶ For if our heart condemn us, God is greater than our heart, and knoweth all things.
 21 Beloved, if our heart condemn us not, then have we confidence toward God.
 22 And whatsoever we ask, we receive of him, because we keep his commandments, and do those things that are pleasing in his sight.
 23 ¶ And this is his commandment, That we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment.
 24 And he that keepeth his commandments dwelleth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he hath given us. (KJV)

Verse 9 says, " 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."   So does that mean if a person sins after they are saved that they were never saved or that they lose their salvation? NO! It has been said before and I will say it again - the best commentary on the Bible is the Bible. To help understand these verses lets look at some verses from Romans 7 & 8. 

I think we can agree that Paul was saved and definately living for God, but even Paul knew what it was for the flesh and the spirit to war. He said in 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.
 16 If then I do that which I would not, I consent unto the law that it is good.
 17 Now then it is no more I that do it, but sin that dwelleth in me.
 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.
 19 For the good that I would I do not: but the evil which I would not, that I do.
 20 Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.

You see the moment that we repent of our sin and trust the Lord Jesus Christ alone and His finished work for our salvation, we are born again. Our spirit that was dead in trespasses and sin (Eph. 2:1) is quickened - that is - made alive. We are HIS seed and He makes us alive. His seed in us does not sin - that is what it is talking about in 1 John and what Paul is talking about in Roman 7. Our new nature hates sin and will not enjoy living in sin. As God's child if we do not get that sin right, God will chastise us and that is a scary thing and can result in much pain in our lives. It is not okay to live in sin. But when we fall and are sorry for it, we can rest assured that He will forgive us. Paul goes on to say in Romans 8:1, "There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit." 

If you don't know Jesus as your Saviour, come to Him today. Believe Him and trust Him as your Saviour. If you do know Him - rest in Him.