1 John 3:5 says, And ye know that he was manifested to take away our sins; and in him is no sin. Speaking of Jesus Christ, John said that we, those who put their faith in Him, know that He came to take away our sins since He was the perfect sacrifice, having no sin Himself. Though being made like us in all ways, Jesus Christ is the only One to live a perfect life. Some people say He was incapable of sinning, but if that were the case, He would not be like all other people.
Verse six adds, Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him. I believe that this means that after we accept Jesus Christ, we cannot continually live a sinful life and not that we will never sin again. I believe that those who have not seen Him refers to lost people of all ages and not just to those who physically saw Jesus.
Verse seven continues, Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous. John said that we should not be deceived, but that those who live righteously are made righteous by the sacrifice of Jesus Christ on the cross for all our sins. We can only be made righteous by putting our faith in Jesus Christ, the only righteous One.
Verse eight states, 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. John then says that those who continue to live in sin are of the devil, who was the first of God's creation to sin. This made it necessary for Jesus Christ to come and die for our sins, and this has been true from the beginning. I don't believe it means that God knew that mankind was going to sin, but that He gave them freewill, and knew that they could sin.
Verse nine adds, 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. I don't believe this means that Christians can never sin, but that they will not continue to live by the sinful standards of the world but will live by faith and ask God's forgiveness once more if they do sin. Even though all our sins are forgiven if we put our faith in Jesus Christ, we still must acknowledge it if we do sin after that.