You, however, are not earthly but spiritual, since the Spirit of God lives within you. Unless a man has the Spirit of Christ, he does not belong to Christ;
This means that it is not the children born in the course of nature who are God's Children, but it is the children born in fulfillment of the Promise who are to be regarded as Abraham's descendants.
Think what love the Father has shown us in allowing us to be called 'Children of God'; as indeed we are. The reason why the world does not know us is that it has not learned to know him.
And he who lays his commands to heart maintains union with Christ, and Christ with him. And by this we know that Christ maintains union with us--by our possession of the Spirit which he gave us.