When we say that the Trinity is a “mystery,” we mean that the Trinity is inexhaustibly deeper than we can ever comprehend. However, “mystery” doesn’t mean that we can’t know anything. The New Testament and the teaching Church and her tradition offer us rich insights about the Trinity.
The great twentieth-century Jesuit theologian, Karl Rahner, reminded us that the fact that the Trinity consists of three Persons is not just a piece of interesting information but information that makes a vital difference in our lives.
We may recognize the unique ways that the three Persons acted during Jesus’ earthly life but we may fail to appreciate that the three Persons act in unique ways in our own lives. Frequently, we pray to or talk about “God” in general. Some may focus on the Father, others on Jesus and others on the Spirit but each Person has a unique role in our lives.
St. Thomas Aquinas attempted to explain the Trinity in a human way, in as much as it is possible. For instance, the Son and the Spirit come forth from the Father but remain one substance with the Father.
We can’t comprehend this but Thomas offers us a human example. We have a thought in our minds. It is our word that comes from us and is united with us. We love our word because it is ours. When we speak our word, we reveal ourselves to others. Others know us though our word. In addition to our word, we have love by which we love others even as the love remains within us.
The Word of God eternally comes forth from the Father and the Father loves the Word. The Father not only brings forth a Word but the Father in loving the Word loves us through His Spirit of Love.
According to St. Thomas, the Father creates each one of us through His Word and through His Love:
God is the cause of things through His mind and will, like an artist of works of art. An artist works through an idea conceived in His mind and through love in His will bent on something. In like manner, God the Father wrought the creature through His Word, the Son, and through His Love, the Holy Spirit.
Thomas explains that our coming forth from the Father reflects the coming forth of the Son and the Spirit. Each one of us comes forth from the Father through His Son, His Word, and through His Spirit, His Love.
In fact, according to Thomas, we not only come forth from the Father through the Son and the Spirit but we go to the Father through the Son and through the Spirit. The very end of our lives reflects our beginning: “Just as we have been created by the Son and the Holy Spirit, so are we united by them to our final end.”  In fact, declares Thomas: “The purpose and the fruit of our whole life is the knowledge of the Trinity in unity.”
The Opening Prayer for the Solemnity of Trinity Sunday thanks the Father “for sending us the Word of Truth and the Spirit of Sanctification.” Of course, we mean the historical sending of the Word in His earthly life but the Father also sends the Word of Truth into our lives, even as the Spirit of Love sanctifies us.
The reading from Paul’s Letter to the Romans, (Rom 5:1-5) gives some expression to the actions of the Three Persons in our lives. The reading proclaims: “We have peace with God through our Lord Jesus Christ through whom we have gained access by faith.” Jesus, by His death and Resurrection, unites us to the Father, now. Paul concludes: “The love of God has been poured into our heart through the Holy Spirit that has been given us.” The Father’s gift of the Spirit was not a one-time event but the Father continues to give us His love through the Spirit.
In the Gospel, (Jn 16:12-15) as Jesus prepares the apostles for His departure, by promising them that the Spirit will bring them understanding of what He has taught: “The Spirit if Truth… will lead you to all truth”
Today, Jesus, the Word teaches us and the Spirit moves our hearts to understand what the Son reveals even as the Spirit helps us to love the Son.
When the Scriptures are read, the Father speaks to us through His Word. The Spirit moves our minds to understand what is being showed to us and also moves our hearts to love the Son whom we receive.
You’ll notice that before the priest says the words of consecration at Mass, he extends his hands over the gifts and asks the Father to send His Holy Spirit that the gifts might become the Body and Blood of Christ. After the words of consecration, the priest prays that we may all be united in Christ by the Holy Spirit. The Spirit enables us to love the Son and to love each other as well.
Each day, the Father gives us His Son as our Way to Him and the Father gives us His Spirit that we might perceive and love the Father through the Son.
Denis Vincent Wiseman, O.P.
 Thomas Aquinas, Summa Theologiae, 1a. 45, 6, in Summa Theologiae, vol. 8, trans. Thomas Gilby, O.P., (London: Blackfriars, 1967), 53.
 1 Sent. d. 14, q. 2 a. 2 in Jean-Pierre Torrell, O.P., Saint Thomas Aquinas: Spiritual Master, trans. Robert Royal (Washington, D.C.: The Catholic University of America Press, 2003), 60.
 Thomas Aquinas, I Sent. 2, I, quoted by Ceslaus Velecky, O.P., in Thomas Aquinas, Summa Theologiae, vol. 6 (London: Blackfriars, 1965), xx.