“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.’ " (Catechism of the Catholic Church 234)

To commemorate Trinity Sunday, here is a selection of five books that previous guests have recommended.

  1. On God and Christ: The Five Theological Orations and Two Letters to Cledonius
    by St. Gregory Nazianzus
  2. The Wellspring of Worship
    by Fr. Jean Corbon
  3. Three Treatises on the Divine Images
    by St. John of Damascus
  4. Called to Communion: Understanding the Church Today
    by Joseph Ratzinger
  5. The Trinity: An Introduction to Catholic Doctrine on the Triune God
    by Gilles Emery OP
Five Books for Catholics may receive a commission from qualifyng purchases made using the affliate links in this post.

To mark Trinity Sunday last year the interview posted was with Fr. Gilles Emery OP, the leading expert on the Trinitarian theology of St. Thomas Aquinas. In that interview, he discussed his books on the subject, the significance of St. Thomas’s teaching, and some of the contemporary debates.

However, Fr. Emery is not the only guest who has talked about the central mystery of the Christian faith. So have several others, while broaching other subjects. They have even recommended excellent books about the Trinity. So, to commemorate Trinity Sunday this year, it made sense to go through the archive, pick out those books, and repropose them. Here then are five such books that delve into the fundamental mystery of the faith. Hopefully, they will help us deeper our friendship with the three divine persons.


Many books have been written about the mystery of God, one and three. Perhaps the finest summation is that of St. Thomas Aquinas. Naturally enough, it featured prominently in the interview with Fr. Emery, a scholar of his Trinitarian theology. Nevertheless, Fr. Emery argued that St. Thomas’s teaching is illuminating because it is rooted in the Church Fathers.

"What did I find in Aquinas? First, Aquinas’s thought about the mystery of the Trinity is in direct continuation with the Church Fathers. This is not so visible in my publications, but in in the courses I give at the theological faculty in Fribourg, I always begin with a semester-long exploration of the Trinity in the Bible and the Church Fathers. Aquinas’s theology is still in living contact with the theology or doctrine of the Church Fathers. In a sense, he is still one of them. He thinks with them. He relies on them: not just Augustine, but many others too. He has the huge advantage of being consistent with the patristic reading of the Bible. The retrieval of the patristic reading of the Bible could find in Aquinas a helpful bridge. He retrieved it. With his help, we can too."

Fr. Emery was not claiming that we can skip the Fathers and read St. Thomas instead. Quite the contrary.

"When I taught, my goal was to have the students access the sources directly. I had them read Tertullian, Basil of Caesarea’s Treatise on the Holy Spirit, Athanasius, Augustine, or Irenaeus of Lyons directly: in the original Greek or Latin, if possible, or in translation. We have good translations in French with Sources chrétiennes. My goal has always been to send the students directly to the sources themselves and not to textbooks. Ironically, I have written textbooks during my professional career. However, I think that students and even theologians should read the sources directly, not through textbooks or secondary literature."

The advice that both St. Thomas and Fr. Emery give, therefore, is redite ad patres: go back to the Church Fathers. Following this advice, the first recommended reading should be from one of the Church Fathers. Fr. Emery mentioned some. So far none of these has made the shortlists of the other guests. One guest, however, did recommend a set of important patristic writings on the Trinity that Fr. Emery did not mention: a series of orations by one of the most important Fathers and doctors of the Church, St. Gregory of Nazianzus.

The five discourses in question (nn. 27-31) have become known as the Theological Orations. They are so-called because they discuss God himself (theologia) rather than the economy of salvation (oikonomia) which plays out in creation (see Catechism of the Catholic Church 236). Moreover, Gregory’s teaching about the Trinity is so authoritative that he is one of three saints that the Orthodox churches call the theologian. The other two are the apostle John and Symeon the New Theologian. St. Gregory of Nazianzus was awarded the title as early as 451, when the Acts of the Council of Chalcedon already refer to him as the Theologian.

The five Theological Orations were given at Constantinople around 379-380. In 379 Theodosius became emperor of the eastern part of the Roman Empire, succeeding Valens, who had fallen the year before at the Battle of Adrianople.

Gregory had moved to the imperial capital under Valens, right when Arianism held sway there, with the emperor’s support. Theodosius, on the other hand, was Pro-Nicene. His ascent to the throne opened an opportunity for Gregory to address the public more openly about the errors of prevalent strand of Arianism—Anomoeanism—and instruct them about who God really is. The Theological Orations are Gregory’s pastoral response to the influential yet heretical teaching of Eunomius of Cyzicus. In them, Gregory is not only catechizing the Pro-Nicene faithful and but also bringing the heterodox back to the true faith.

Anomoeanism had been started by Aëtius of Antioch but its main exponent was his follower, Eunomius of Cyzicus. Proof thereof is that each of the three Church Fathers known as the Cappadocians wrote to debunk the latter’s writings. There is, example, Basil the Great’s Against Eunomius, Gregory of Nyssa’s Against Eunomius, and Gregory Nazianzen’s Theological Orations.

Whereas the Council of Nicaea had declared that the Son is of the same nature or essence (homoousios) with the Father, Eunomius denied its teaching and held that the Father alone was truly God. He even rejected the position of those Arians who sought to hijack and compromise Nicaea by proclaiming the Son to be similar in nature (homoiousios) to the Father rather than consubstantial with him. He insisted that the Son and the Spirit were not at all similar in essence to the Father. If we do call them divine, it is not in the strict sense of the term. Eunomius relied heavily on dialectic, or rather its abuse, to convince people that the traditional reading of Scripture and faith in the Trinity were unsound. This rationalist streak was so marked that he even claimed to know God as God knew himself.

Gregory, on the other hand, shows how such rationalism is unwarranted. On the one hand, it wrongly supposes that we can think about God the way we would about any other matter. That is not the case. Prayer and purification are needed. On the other hand, the rationalist wrongly supposes that our limited intellect can know in full the one who transcends all creation when it cannot even fully grasp the essence of creatures. Consequently, the rationalist fails to appreciate that saying something about God works differently from saying something about a creature. To say something about God we use the words coined to refer to things in the world around us but always on the understanding that God differs radically form the creature. We mean that God is such-and-such, but not in the way that a creature is. Appreciating this is crucial for a correct understanding of the words Scripture uses to describe God: the Father, the Son, and the Spirt. However, that is precisely what Sabellians, Arians, and Anomoeans fail to appreciate. Much of the Theological Orations is taken up with exposing their fallacious semantics and explaining what Scripture really says about God. Moreover, Gregory turns Eunomius’s dialectical method against him and dialogues with his congregants in question-and-answer format. He thereby shows that they are capable of reasoning their way to the right answers on these challenging questions and that Eunomius, far from being a master dialectician, is a rationalist guilty of sophistry.

“The aim is to safeguard the distinctness of the three persons within the single nature and quality of the Godhead. The Son is not Father; there is one Father. Yet he is whatever the Father is. The Spirit is not Son because he is from God; there is one Only-begotten. Yet whatever the Son is, he is. The three are a single whole in their Godhead and the single whole is three in its individual distinctions.” Gregory of Nazianzus

The theological debates have changed but the issues that Gregory addresses in the Theological Orations are fundamental and perennially important. Though the subject matter is challenging, the Theological Orations are also full of spiritual guidance. They have been published in the Popular Patristics series. The same translation is also available with a detailed commentary in Faith Gives Fullness to Reasoning: The Five Theological Orations of Gregory Nazianzen.


This post is for paying subscribers only

Sign up now and upgrade your account to read the post and get access to the full library of posts for paying subscribers only.

Sign up now Already have an account? Sign in