Dear Brothers and Sisters,

1. "A single font and a single root, a single form filled with the triple splendor. There where the profundity of the Father shines, breaks forth the power of the Son, wise creator of the entire universe, fruit generated from the fatherly heart! And there shines out the unifying light of the Holy Spirit." Sinesius of Cyrene sang these words at the beginning of the 5th century in Hymn II, celebrating the Holy Trinity at the dawn of a new day, as one in source and triple in splendor. This truth of the one God in three persons, equal yet distinct, is not limited to the heavens; it cannot be interpreted as some sort of "heavenly arithmetic theorem" from which nothing comes for the existance of humanity, as the philosopher Kant supposed.

2. In fact, as we heard in the account of Luke the Evangelist, the glory of the Trinity is made present in time and space and finds its highest manifestation in Jesus, in His incarnation, and in His story. The conception of Christ was read by Luke in the light of the Trinity: it is the words of the angel that attest to this, words directed to Mary and pronounced within a humble house of the Galilean village of Nazareth, which has been discovered by archeology. In Gabriel's announcement, the trancendent divine present is made manifest: the Lord God, through Mary and in the line of David's descendents -- gives the world His Son: "And behold, you will conceive in your womb and bear a son, and you shall call His name Jesus. He will be great, and will be called the Son of the Most High; and the Lord God will give to Him the throne of His father David" (Lk 1:31-32).

3. The word "son" has two meanings here, because in Christ the filial link with the Heavenly Father and that with the earthly mother are intimately linked. But in Holy Spirit also takes part in the Incarnation, and it is His intervention that makes that generation unique and irrepeatable: "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God" (Lk 1:35). The words that the angel proclaims are like a small Creed, which sheds light on the identity of Christ in relation to the other Persons of the Trinity. It is the choral faith of the Church, which Luke already asserts at the start of the time of the salvific fullness: Christ is the Son of the Most High God, the Great One, the Holy One, the King, the Eternal One, Whose generation in the flesh is completed by the work of the Holy Spirit. Thus, as John says in his First Letter, "No one who denies the Son has the Father. He who confesses the Son has the Father also" (1 Jn 2:23).

4. The Incarnation stands at the center of our faith. In it the glory of the Trinity and His love for us are revealed. "The Word was made flesh and dwelt among us, and we have beheld His glory" (Jn 1:14). "God so loved the world that He gave His only begotten Son" (Jn 3:16). "In this the love of God was made manifest among us, that God sent His only Son into the world, so that we might live through Him" (1 Jn 4:9). Through these words of the Joannine writings we are able to comprephend how the revelation of the glory of the Trinity in the Incarnation is not just a simple illumination that tears through the darkness for an instant, but rather a seed of divine life deposited forever in the world and in the hearts of men and women.

    A declaration of the Apostle Paul in the Letter to the Galatians is emblematic here: "But when the time had fully come, God sent forth His Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of His Son into our hearts, crying, 'Abba! Father!' So through God you are no longer a slave but a son, and if a son then an heir" (Gal 4:4-7, cf. Rm 8:15-17). The Father, Son, and Spirit are therefore present and act in the Incarnation to bring us into their own life. "All people," confirmed Vatican Council II, "are called to this union with Christ, Who is the light of the world; we come from Him, we live through Him, we are directed toward Him" (LG, n. 3). As St. Cyprian affirmed, the community of the children of God is "a people assembled by the unity of the Father, the Son, and the Holy Spirit" (De Orat. Dom. 23).

5. "To know God and His Son is to accept the mystery of the loving communion of the Father, the Son and the Holy Spirit into one's own life, which even now is open to eternal life because it shares in the life of God. Eternal life is therefore the life of God Himself and at the same time the life of the children of God. As they ponder this unexpected and inexpressible truth which comes to us from God in Christ" (Evangelium vitae, nn. 37-38).

    In this stupor and in this vital acceptance, we must adore the mystery of the Most Holy Trinity, which "is the central mystery of Christian faith and life. It is the mystery of God in Himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them" (Cathechism of the Catholic Church, n. 234).

    In the Incarnation, we contemplate the Trinitarian love that unfolds itself in Jesus; a love that does not remain closed in a perfect circle of light and glory, but radiates itself in the flesh of men and women, in their history; it pervades men and women, regenerating them and making them children of the Son. For this reason, as St. Irenaeus said, the glory of God is the living person: "Gloria enim Dei vivens homo, vita autem hominis visio Dei"; this is so not only for his physical life, but especially because "the life of a person consists in the vision of God" (Adversus Haereses IV, 20, 7). For seeing God transfigures us into Him. "We shall be like Him, for we shall see Him as He is" (1 John 3:2).