According to a famous legend, Saint Patrick is said to have used a shamrock to teach the doctrine of the Trinity when evangelizing Ireland in the 5th century. (The lovely window of Saint Patrick in our church is one of the few depictions of him not holding a shamrock!)The fact that Patrick had to resort to using a shamrock illustrates our awkwardness when talking about the Trinity. The problem perhaps is not so much that the Trinity is a supernatural mystery, which we can never completely understand, but rather that it seems so hard to connect with experientially. It seems abstract, more like a philosophical idea than an expression of our regular religious experience.
And yet, as ideas go, it is inseparable from our religious experience as Christians. All of us, after all, were baptized in the name of the Father, and the Son, and the Holy Spirit. On that occasion, we – or our parents and godparents speaking on our behalf - all made a profession of faith in the Father, and the Son, and the Holy Spirit. My sins have been forgiven many times, in the name of the Father, and of the Son, and of the Holy Spirit. And as a priest I have been privileged many times to absolve the sins of others - in the name of the Father, and of the Son, and of the Holy Spirit. I have officiated at weddings – at which rings have been 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 – so many times in the name of the Father, and of the Son, and of the Holy Spirit. We began Mass this morning with the Sign of the Cross, in the name of the Father, and the Son, and the Holy Spirit, and we will end it with a blessing, in the name of the Father, and the Son, and the Holy Spirit. The principal prayers of the Church’s liturgy are all explicitly addressed to the Father, through the Son, in the unity of the Holy Spirit. In short, our entire lives in the Church have been defined, formed, and shaped by the awesome mystery of who God is, that 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, that may be because the Holy Trinity seems to surround us all the time.
So for all its apparent abstraction, the doctrine of the Trinity is our fundamental – and uniquely Christian – insight into who God is. Created in the image and likeness of God, we all have a built-in natural longing for God. So we can theorize about God’s existence by our ordinary, natural reasoning process. But who God is - in himself - that is something we cannot possibly know on our own. That had to be revealed to us.
Even so, the words we use to speak about God’s Trinitarian self - for example, the language of the Nicene Creed that we sing every Sunday at Mass – that language is very much the product of a process of reasoned discussion and debate in the early Church. We call it the Nicene Creed in memory of the 1st Ecumenical Council of the Church, held at Nicaea (in what is today Turkey) in 325, at which the famous “318 Holy Fathers” put the Church’s Trinitarian faith into official language. (The Creed we recite at Mass is actually the product of the first two ecumenical councils – Nicaea in 325 and Constantinople in 381 – and is officially called the “Niceno-Constantinopolitan Creed” or “Nicene Creed,” for short).
The issue at Nicaea in 325 was the so-called Arian heresy which denied the divinity of Christ. It was in response to this that the Creed was composed, formally articulating the Church’s faith in who God is. After that, devotion to the Holy Trinity developed in the Church and increased throughout the Middle Ages. Eventually in 1334, Pope John XXII established this feast for the entire Church and directed that it be celebrated annually on the Sunday after Pentecost.
We do not worship three gods, but one God – a unity of Persons in one divine nature or substance. Each of the three Persons, Father, Son, and Holy Spirit, is truly God. They are distinct. – not three modes or appearances of God but three identifiable Persons, each distinctly God, but inherently in relationship to each other. That is to say, their distinction resides in their relationship to each other: the Father to the Son, the Son to the Father, and the Holy Spirit to both.
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. Who God is in himself is how God acts. And so how God acts reveals who God is.
Already in the Old Testament, God was revealing himself – as he did to Moses in today’s 1st reading, as one who reveals himself 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.
God’s action outside himself is the indivisible, common work of the Trinity of Persons, which each Person performs according to his unique personal property. Thus the 2nd Ecumenical Council of Constantinople  confessed: “one God and Father from whom all things are, and one Lord Jesus Christ, through whom all things are, and one Holy Spirit in whom all things are.”
It is, of course, the Son, consubstantial with the Father, who, as the visible image of the invisible God, came down from heaven, so that the world might be saved through him. 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 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 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.
So it is no merely theoretical abstraction that God's grace is given to us from the Father, through the Son, in the Holy Spirit. As the 4th century Bishop and Doctor of the Church, St. Athanasius, famously 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.”
Hence, the Church faithfully follows Saint Paul in praying: The grace of our Lord Jesus Christ, and the love of God, and the communion of the Holy Spirit be with you all!
Homily for Trinity Sunday, Immaculate Conception Church, Knoxville, TN, June 11, 2017