Behold what manner of love the Father has given to us, that we should be called children of God. And that is what we are! The reason the world does not know us is that it did not know Him.
John calls believers to behold the Father's love in making them His children and explains that the world does not know them because it did not know God.
This verse opens the section by grounding Christian identity in divine love.
1Behold what manner of love the Father has given to us, that we should be called children of God. And that is what we are! The reason the world does not know us is that it did not know Him. 2Beloved, we are now children of God, and what we will be has not yet been revealed. We know that when Christ appears, we will be like Him, for we will see Him as He is. 3And everyone who has this hope in Him purifies himself, just as Christ is pure. 4Everyone who practices sin practices lawlessness as well. Indeed, sin is lawlessness. 5But you know that Christ appeared to take away sins, and in Him there is no sin. 6No one who remains in Him keeps on sinning. No one who continues to sin has seen Him or known Him. 7Little children, let no one deceive you: The one who practices righteousness is righteous, just as Christ is righteous. 8The one who practices sin is of the devil, because the devil has been sinning from the very start. This is why the Son of God was revealed, to destroy the works of the devil. 9Anyone born of God refuses to practice sin, because God’s seed abides in him; he cannot go on sinning, because he has been born of God. 10By this the children of God are distinguished from the children of the devil: Anyone who does not practice righteousness is not of God, nor is anyone who does not love his brother.