1John 3:1 How great is the love the Father has lavished on 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.
It was gracious enough that Christ died for our sins so as to reconcile us to God, but to go beyond that and give us the status of children of God is really over the top! Of course sonship is limited to those who have faith in Christ. "You are all sons of God through faith in Christ Jesus" Gal 3:26 And "to all who received him, to those who believed in his name, he gave the right to become children of God" John 1:12
"Now if we are children, then we are heirs— heirs of God and co-heirs with Christ" Rom 8:17