All that is in spite of the obvious fact that our religious lives are all thoroughly permeated by the language of the Trinity. All of us were baptized in the name of the Father, and the Son, and the Holy Spirit. On that occasion, we – or our parents and godparents - all made a profession of faith in the Father, and the Son, and the Holy Spirit. We begin every Mass and most of our prayers with the Sign of the Cross – in the name of the Father and of the Son and of the Holy Spirit. The Collect at Mass is addressed to the Father through the Son in the unity of the Holy Spirit. When we pray the psalms, we conclude each psalm with Glory to the Father and to the Son and to the Holy Spirit, and we do the same with each decade of the rosary. Our sins have been forgiven in confession, in the name of the Father, and of the Son, and of the Holy Spirit. At weddings, rings are exchanged in the name of the Father, and of the Son, and of the Holy Spirit. We have all been blessed – and have blessed ourselves - in the name of the Father, and of the Son, and of the Holy Spirit. In short, both individually and as a Church community, we have been defined, formed, shaped by the awesome mystery of who God is, which defines the Triune God’s relationship with us and ours with God. If we seem sometimes to take the idea of the Trinity for granted, it may be because it seems to surround us all the time.
Our profession of faith in the Trinity, as Walter Kasper [The God of Jesus Christ, Continuum, 2012] has written, "is the Christian form of speaking about God, that at the same time claims to express the eschatologically definitive and universal truth about God from which alone all other talk about God can derive its full truthfulness. It is the objective and even objectively necessary and binding formulation of the eschatological revelation God has given of himself in Jesus Christ through the working of the Holy Spirit. ... The doctrine has its basis solely in the history of God's dealings with human beings and in the historical self-revelation of the Father through Jesus Christ in the Holy Spirit."
On the one hand, the doctrine of the Trinity expresses our uniquely Christian insight into the inner life of God – where the Son is the image of the Father, the Father’s likeness and outward expression, who perfectly reflects his Father, while the Holy Spirit in turn expresses and reveals the mutual love of Father and Son. At the same time, the Trinity also expresses something fundamental about how God acts outside himself. In terms of our religious experience, it is how God acts that reveals who God is.
Already in the Old Testament, God was revealing himself as one whose nature is revealed in how he acts toward us: a merciful and gracious God, slow to anger and rich in kindness and fidelity. It was to such a God that Moses prayed – as we all pray – do come along in our company … and receive us as your own [Exodus 34:6, 8]. It is, of course, the Son, consubstantial with the Father, who, as the visible image of the invisible God, came into the world, so that the world might be saved through him [John 3:17]. Through him we have peace with God and have gained access by faith to this grace in which we stand [Romans 5:1-2]. Risen from the dead and seated at the right hand of the Father, the Son has sent the Holy Spirit upon his Church, which is the Body of Christ and the Temple of the Holy Spirit. The Holy Spirit unites us with the Father in the Body of Christ, the Church. Through the sacraments, Christ continues to communicate the Holy Spirit to the members of his Church. The Holy Spirit is inseparable from the Father and the Son, in both the inner life of the Trinity and his gift of love for the world. The Holy Spirit’s presence in us enables us to experience the presence and action of God in our lives not as an abstraction but as a real relationship.
So it is no merely theoretical abstraction that God's grace is given to us from the Father, through the Son, in the unity of the Holy Spirit. As the famous 4th-century Bishop and Doctor of the Church, St. Athanasius, wrote in one of his letters: When we share in the Spirit, we possess the love of the Father, the grace of the Son, and the fellowship of the Holy Spirit himself.