The Trinity is the Church’s teaching about who God is in himself: one God in three persons. Every other doctrine of the faith, from creation to the sacraments, is received in its light.
The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity". The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God." In the words of the Fourth Lateran Council (1215), "Each of the persons is that supreme reality, viz., the divine substance, essence or nature."
The divine persons are really distinct from one another. "God is one but not solitary." "Father", "Son", "Holy Spirit" are not simply names designating modalities of the divine being, for they are really distinct from one another: "He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son." They are distinct from one another in their relations of origin: "It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds." The divine Unity is Triune.
The divine persons are relative to one another. Because it does not divide the divine unity, the real distinction of the persons from one another resides solely in the relationships which relate them to one another: "In the relational names of the persons the Father is related to the Son, the Son to the Father, and the Holy Spirit to both. While they are called three persons in view of their relations, we believe in one nature or substance." Indeed "everything (in them) is one where there is no opposition of relationship."
Is God one, and how does divine revelation speak of the Father, the Son and the Holy Spirit?
The words of Deuteronomy are Israel’s confession that the Lord is one. The Church receives them as the revelation of the one God, on which the New Testament’s revelation of the Trinity rests and which it does not replace. It bears on the unity of God.
Hear, O Israel, the Lord our God is one Lord.
In 1 Corinthians, writing on spiritual gifts, Paul refers the diversity of graces, ministries and operations to the same Spirit, the same Lord and the same God. The three are named as one source of the Church’s life. It bears on one God and the three who act together.
Now there are diversities of graces, but the same Spirit. And there are diversities of ministries, but the same Lord. And there are diversities of operations, but the same God, who worketh all in all.
Paul closes 2 Corinthians with a blessing that names the Lord Jesus Christ, God and the Holy Ghost together. It is among the earliest places where the three stand side by side in the Church’s worship. It bears on the Trinity confessed in the Church’s prayer.
The grace of our Lord Jesus Christ and the charity of God and the communication of the Holy Ghost be with you all.
At the close of Matthew the risen Christ commands the apostles to baptize in the name of the Father, and of the Son, and of the Holy Ghost. One name is joined to three persons, and this became the baptismal form of the Church. It bears on the Trinity confessed in baptism.
Going therefore, teach ye all nations: baptizing them in the name of the Father and of the Son and of the Holy Ghost.
The baptism of Jesus in Matthew presents the Son in the Jordan, the Spirit descending, and the Father’s voice from heaven. The Fathers often cite this theophany for the three persons, distinct and manifested together. It bears on the real distinction of the persons.
And Jesus being baptized, forthwith came out of the water: and lo, the heavens were opened to him: and he saw the Spirit of God descending as a dove, and coming upon him. And behold a voice from heaven saying: This is my beloved Son, in whom I am well pleased.
The prologue of John declares that the Word was with God and was God, and that the Word was made flesh. It is the principal text of Scripture for the divinity of the Son and for his distinction from the Father. It bears on the Son as God, equal with the Father.
In the beginning was the Word: and the Word was with God: and the Word was God. The same was in the beginning with God. All things were made by him: and without him was made nothing that was made.
And the Word was made flesh and dwelt among us (and we saw his glory, the glory as it were of the only begotten of the Father), full of grace and truth.
With these words the prologue of John ends: the only begotten Son, who is in the bosom of the Father, has declared him. The Fathers read it for the Son’s origin from the Father and his sharing in the Father’s nature. It bears on the divinity of the Son and his generation.
No man hath seen God at any time: the only begotten Son who is in the Bosom of the Father, he hath declared him.
In John the Lord says that he and the Father are one. The Fathers read “one” as the unity of nature and “are” as the plurality of persons, as Tertullian does in the passage quoted below. It bears on the one God and on the divinity of the Son.
I and the Father are one.
In the farewell discourse of John, the Lord promises another Paraclete, the Spirit of truth, whom the Father will send in his name. The Church reads the promise as speaking of a divine person distinct from the Father and the Son. It bears on the Holy Spirit’s place beside them.
And I will ask the Father: and he shall give you another Paraclete, that he may abide with you for ever: The spirit of truth, whom the world cannot receive, because it seeth him not, nor knoweth him. But you shall know him; because he shall abide with you and shall be in you.
But the Paraclete, the Holy Ghost, whom the Father will send in my name, he will teach you all things and bring all things to your mind, whatsoever I shall have said to you.
In John the Lord speaks of the Spirit of truth who proceeds from the Father, and says that the Spirit receives of what is his. The Latin tradition cites these words in the doctrine of the procession of the Holy Spirit. It bears on his procession from the Father and the Son.
But when the Paraclete cometh, whom I will send you from the Father, the Spirit of truth, who proceedeth from the Father, he shall give testimony of me.
All things whatsoever the Father hath are mine. Therefore I said that he shall receive of me and shew it to you.
Thomas’s words to the risen Christ in John are an address to him as God. The Lord does not correct them, and the Church receives them as apostolic witness to his divinity. It bears on the Son as God.
Thomas answered and said to him: My Lord and my God.