Something of what the Holy Father said, in Italian, during his Sunday Angelus address:
Baptism suggests very well the global meaning of the Christmas season, in which the theme of "becoming children of God" thanks to the coming of the only-begotten Son in our humanity constitutes the dominant element. He became man so that we could become the children of God. God was born so that we might be reborn. … This stupendous mystery which is our "second birth" – the rebirth of a human being from "on high", from God (cf John 3:1-8) – is realized and summed up in the sacramental sign of Baptism.
With this kind of a sacrament man becomes truly "son", son of God. Thence, the goal of his existence consists in reaching in a way both free and aware that which from the very beginning he received as a gift. "Become that which you are" – represents the educative principle at the foundation of the human person redeemed by grace. [More on this below.] Such a principle has many analogies with human growth, where the relation between parents with children passes, through disconnects and crises, from total dependence to the awareness of being children, to recognition of the gift of life received and the maturity and capacity to give life. Generated from Baptism into new life, also the christian begins his journey of growth in the faith which will bring him to invoke in awareness God as "Abba – Father", and to turn to Him with gratitude and to live the joy of being His son.
From Baptism there derives also a model for society: that of brothers. Brotherhood can’t be established through an ideology, and even less by decree constituted by any power. We recognize each other as brothers from the humble but deep awareness of one’s own being sons of the unique heavenly Father. As Christians, thanks to the Holy Spirit received in Baptism, we share the lot of the gift and duty to live as children of God and as brothers, to be as "leaven" of a new humanity, in solidarity and rich with peace and hope. We are helped in this by the awareness of having, in addition to a Father in heaven also a mother, the Church, of whom the Virgin Mary is the perennial model. To her do we entrust these newly baptized babies and their families, and we ask for all the joy of being reborn every day "from on high", from the love of God, which makes us his children and, amongst ourselves, brothers.
"Become that which you are".
In the Pastoral Constitution on the Church in the Modern World, Gaudium et spes 22 – which I think is in part a key to the Holy Father’s remarks – we read:
The truth is that only in the mystery of the incarnate Word does the mystery of man take on light. For Adam, the first man, was a figure of Him Who was to come, namely Christ the Lord. Christ, the final Adam, by the revelation of the mystery of the Father and His love, fully reveals man to man himself and makes his supreme calling clear.
We are created in God’s image and likeness. In the fullness of time, the perfect invisible image of the invisible Father took our humanity into an indestructible bond with His divinity and became the perfect visible image of the invisible Father. When we look to the World made flesh, we learn who we really are. He is not only a paradigm or a model after which we fashion our words and acts. We are incorporated into His mystical Person through the sacrament of baptism. Our words and deeds can be more through our indestructible bond in Him. By baptism and our awareness of its gifts, we come to be that which we truly are.
You might take a few minutes today and read Gaudium et spes 22-25.