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.
That we should be called children of God ...  The essential kinship of humanity with the Creator is glimpsed in such a concept as this, as it is seen also in the great truth underlying the doctrine of the Incarnation. God would not have become a man, unless it had been true that man had been created in God's image.  The most glorious truth the world has ever received is in this invitation or "call" of God to become his children.