Sunday is the Solemnity of the Most Holy Trinity. It’s so easy to take that for granted — to listen to Jesus in the Gospel, speaking about the Spirit and about his Father, with the ears of someone who has heard that old story s-o-o-o many times before, then to drive back home, mow the lawn and enjoy your Sunday dinner and maybe a movie.

But wait! What is “three persons in one God” really about?

The word “Trinity” never shows up in the Bible, but with the eyes of faith, you can find it all over the place. Remember when Jesus was baptized by John in the Jordan River — and God the Father spoke from the clouds, saying, “This is my beloved Son in whom I am well pleased.” The Holy Spirit was there too, manifesting himself as a dove.

Then in Matthew 28:19, Jesus instructed the disciples, telling them, “Go and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit.”

In “St. Patrick’s Breastplate,” his morning prayer, St. Patrick prays:

I bind unto myself today

The strong Name of the Trinity,

By invocation of the same,

The Three in One and One in Three.

The Catechism of the Catholic Church (nos. 233-237) explains this great mystery, which is held true not only by Catholics but by all Christians:

“The mystery of the Most Holy Trinity is the central mystery of Christian faith and life... It is therefore the source of all other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the hierarchy of the truths of faith. The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son, and Holy Spirit, reveals Himself to Men and reconciles and unites with Himself those who turn away from Him.

“The Trinity is a mystery of faith in the strict sense, one of the mysteries that are hidden in God, which can never be known unless they are revealed by God. To be sure, God has left traces of His Trinitarian being in His work of creation and in His Revelation throughout the Old Testament. But his inmost Being as Holy Trinity is a mystery that is inaccessible to reason alone or even to Israel’s faith before the Incarnation of God’s Son and the sending of the Holy Spirit.”

Glory be to the Father, and to the Son, and to the Holy Spirit!