Chapter 3 of John's First Epistle opens,
See what love the Father has given us, that we should be called children of God; and so we are. The reason why the world does not know us is that it did not know him.
But why do we not know us? That's the real question. Oh yes, in a superficial way we know that we are God's children. Some well intended people will say, "We are all God's children." But the minute you say that every person is a "child of God" then you have just robbed the phrase of all meaning. It's like our tendency to give every child a prize.
No, when St. John speaks of τέκνα θεού (children of God) he does not simply mean human being. For John there is a bright line that separates the world (kosmos) from the children of God.
Our fundamental human dignity comes from being created in the image and likeness of God (Genesis). Everyone has that dignity from the moment they are conceived.
Sperm+egg+soul=human with inviolable dignity.
But that human being is not a child of God.
For St. John a child of God is a human being who has been radically transformed by God's grace. With the birth of Jesus we have the ultimate step in human evolution. To use a metaphor we might understand, think of a child of God as a new species. The Child of God is more different than Homo Sapiens is from his ancestor Homo Erectus.
The child of God is a new being. The problem is that from the outside the child of God looks like your ordinary human being. And because we rarely tap our potential the children of God often behave like ordinary human beings.
St. John says that the reason the world cannot tell us apart is that they do not know Christ. And perhaps this is the same reason that we who are children of God can tell ourselves apart from the rest of the world. We do not know Christ well enough. We do not grasp how radically the incarnation changed the world.
In America we worship equality. But Christianity is precisely about not being equal, not being like everyone else. If we choose to be like everyone else then Christ died for nothing, the birth of Christ was meaningless.
We read the letters of John in this Christmas season to remind of who we are, who we have become by God grace. Only when we embrace our true identity as children of God will we tap into the power of Holy Spirit dwelling in us. We are meant to do great things if we would only believe. As Jesus tells us in John's gospel,
Very truly I tell you, whoever believes in me will do the works I have been doing, and they will do even greater things than these.
That's what it means to be children of God.