Behold, how great a love the Father has bestowed on us, that we should be called children of God! For this cause the world doesn't know us, because it didn't know him.
Unbelievers cannot fully comprehend the children of God. The reason for this lack of perception is their failure to comprehend God fully. Since they do not "know" the Parent they do not "know" the children (cf. John 1:12-13; John 5:37; John 7:28; John 16:3).