If I had a dollar for every priest likely to launch his Trinity Sunday sermon with, “This is a mystery,” I would be a rich man. The Blessed Trinity is a mystery, but that does not mean it lacks significance for us. But I think it was the German theologian Karl Rahner who once observed that, if the Holy Trinity were to disappear, most Catholics would not even notice.

Well, they should.

What does the Trinity have to say to us?

The Trinity is a communio personarum, a “communion of persons.” In God, the problem of unity and diversity is overcome in a way man will never achieve: one Godhead, yet three Persons.

Amid all the sophisticated distinctions of Trinitarian and Christological theology, the mystery of “begettings” and “processions” can be summarized in a few words: God loves. God loves with the fullness of his Being. And because love necessarily involves another, God’s love gives life: the Father gives life to the Son, who is as truly and eternally and perfectly God as the Father. And the Son, the object of the Father’s love, loves eternally, so that where their loves join proceeds the Spirit, who proceeds from the love of “the Father and the Son.”

God’s love is always open to life, is always life-giving. God’s openness to life is not some theoretical positive thinking. It is fertile. It is life-giving. It gives rise to Persons.

And who is man? Man is God’s “image and likeness” (Genesis 1:28). Man is created in God’s image and likeness—a God who is tri-Personal, a God who is life-giving, a God who is Love.

As St. John Paul II wrote in Familiaris consortio: “God is love and in Himself He lives a mystery of personal loving communion. Creating the human race in His own image and continually keeping it in being, God inscribed in the humanity of man and woman the vocation, and thus the capacity and responsibility, of love and communion. Love is therefore the fundamental and innate vocation of every human being.” God made us “through love … and for love” (emphasis added).

God didn’t have to create us. Love, of course, demands a response, but the response has already taken place, in the Trinity. The Father’s Love has a perfect response in the Son, and their Love for each other has a perfect response in the Holy Spirit.

But love has that impulse to expand, to generativity, to creativeness. And so God created, not because He had to, but because He chose to: “all things, visible and invisible.”

But He also chose to create one creature in His Image: man.

So, what does that say about the way man should love, especially in the way he most directly shares in forming a communion of persons—through sexual intercourse, that can give rise to persons?

Humanae vitae addressed that question almost 51 years ago: human love should be “human, total, faithful and exclusive, and fertile.” It is “human,” because it must be what man is: body and soul, a foot in the material world, a foot in the spiritual. And, as the late William May was wont to note, just as Jesus is “begotten, not made,” so should the human child come into the world as an expression of “human” (i.e., spiritual and bodily) union, not laboratory technique (IVF, surrogacy, artificial insemination). Human love must be “total,” a complete self-gift, like God Himself. It must be “faithful and exclusive,” that is, personally exclusive (rejecting polygamy and polyandry) and personally faithful (rejecting divorce as incompatible with indissolubility). And it must be “fertile,” i.e., open to giving life, just as God is always open to giving life.

Furthermore, as St. John Paul II also noted—quoting Ephesians 3:15—all fatherhood comes from God. God is the source of life, and all life-giving capacity is a participation in his life. Also, as John Paul noted, while sexual intercourse has biological consequences—we are, after all, biological creatures, not angels trapped in matter—only God can create a soul. So, every act of fertile sexual intercourse is inherently a three-some: the cooperation of a man and a woman with God who blesses and consecrates their love with life that comes from his hand, from the Father “from whom every family in heaven and on earth takes its name.” God is involved with man from the beginning: as Yahweh reminded various prophets (Isaiah 44:24; Jeremiah 1:5), he knew you in the womb.

So, as you reflect on Trinity Sunday, don’t imagine this feast is some intellectual exercise of little significance to you. It certainly has a lot to say about human life and love – if we are willing to learn its lessons.

All opinions expressed in this essay are exclusively those of the author.