1 John 3:1 says, 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. John said that God showed His love for us by giving us the right to be called the sons of God if we accept His gift of salvation. He then said that if we become a child of God by faith that the world would no longer know us, because the world didn't know Him. I believe this means that they will no longer understand us spiritually and not physically. Our values should set us apart from the world.
Verse two adds, 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. John said that the beloved, those who had accepted Jesus Christ, were children of God and they couldn't know exactly what they would one day become, but when Jesus Christ returned, they would then know. Since the Bible often refers to believers as sons of God, this is why I believe that it is important to state that Jesus is the only begotten Son of God. This also applies to daughters of God as well, even though it usually just refers to sons.
Verse three continues, And every man that hath this hope in him purifieth himself, even as he is pure. John also said that if we put our faith in Jesus Christ that we will purify ourselves from the sins of the world. Though we are purified by the sacrifice of Jesus Christ for our sins, once we accept Him as our Savior and Lord, we should
then work to eliminate sin from our lives.
Verse four concludes, Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law. John said that if we did sin that we transgressed the law of God, because sin is the transgression of God's law. We can break some of man's laws without being guilty of sin, but we cannot break God's law, even in the way we think, without being guilty of sin.