What is the most central doctrine of the Catholic faith?

I’ve asked Catholics that question many times when teaching, and I’ve heard quite a few good guesses. Most frequently, people respond, “the Eucharist” or “the Real Presence.” I’ve also heard someone respond, simply, “Jesus.”

I certainly can’t disagree with the importance of those core aspects of the Catholic faith (especially with ‘Jesus’!), but all Catholic teaching relies on a single foundational truth: the Trinity. The Catechism of the Catholic Church explains:

234 The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the 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 sin.”

In other words, no other Catholic doctrines make sense without the reality of the Trinity. You can’t appreciate the truth that Jesus is present in the Eucharist unless you know who Jesus is, and you can’t appreciate that Jesus is the Son of God unless you understand that there are three divine persons – the Father, the Son, and the Holy Spirit. Every aspect of Catholicism is in some way rooted in the mystery of who God is as an eternal Trinity of divine persons. Since we recently celebrated Trinity Sunday, it seems like an appropriate time to highlight this truth.

The dogma of the Trinity has been very carefully defined by the Catholic Church through the centuries. The First Council of Nicaea (325 AD) wrote the first version of the Nicene Creed, teaching that the Son has the same divine nature as the Father. The First Council of Constantinople (381 AD) concluded the Niceno-Constantinopolitan Creed that we recite each Sunday, affirming that the divine Holy Spirit is “adored and glorified” with the Father and the Son. The Councils of Ephesus (431 AD) and Chalcedon (451 AD) provided further clarifications about the truth that Jesus is both human and divine. If you want to learn about the mystery of the Holy Trinity, the Catechism of the Catholic Church (especially no. 232 – 267) is very helpful.

The average Catholic may wonder, though, why the doctrine of the Trinity really matters. What difference does it make to say things like, “we worship one God in Trinity, and Trinity in Unity, neither confounding the Persons, nor dividing the essence” (Athanasian Creed)?

To put it very simply, you can’t love God if you don’t know him! You don’t need a theology degree to be a saint, but you should have a basic familiarity with the things that God has revealed about himself in Scripture and Tradition.

My favorite paragraph of the Catechism of the Catholic Church, no. 221, offers this explanation of the Trinity:

221 But St. John goes even further when he affirms that “God is love”: God's very being is love. By sending his only Son and the Spirit of Love in the fullness of time, God has revealed his innermost secret: God himself is an eternal exchange of love, Father, Son and Holy Spirit, and he has destined us to share in that exchange.

The truth about the Trinity is not a lifeless theological formula but is the deepest truth about the God who is love. For all of eternity, the Father gives life to the Son, the Son receives life from the Father, and the Holy Spirit is breathed forth as the eternal expression of their infinite love for one another. The divine persons have perfect joy in this eternal exchange of love, and they’ve created us to become children of God and share in the joy of this love. The Holy Trinity is communion of persons, and he’s created us to be in communion with him and with one another.

Last Sunday we celebrated the Solemnity of the Body and Blood and Truth, which highlights the truth of the Eucharist, a truth which is itself rooted in the Trinity. Here’s what I mean: We participate in the Mass and receive the Eucharist in order to be united to Jesus Christ, the Second Person of the Trinity, who unites us to the Father in the Holy Spirit.

There is no truth more beautiful than the truth of the Trinity! At the risk of self-promotion, much of what I’ve written has focused on a Catholic understanding of the Trinity. My master’s thesis, for example, was about how Christ and the Trinity should be at the center of catechesis. My second book, Remain in Me: Scriptural Reflections on Growing as a Disciple of Jesus, offers reflections on Jesus’ deep communion with the Father and our call to share in that communion.

If you want to deepen your understanding and appreciation of the Catholic faith, I encourage you to learn about and meditate on the mystery of the Trinity, because nothing else makes sense without it!