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Veil to Unveil: Aquinas on the Mystery of the Mass

April 18, 2024


Thomistic theology is rarely associated with liturgical prayer, even by many of St. Thomas’ own disciples. Such a dissociation reveals more about the priorities of later Thomism, however, than it does about St. Thomas Aquinas, who himself devoted considerable energy to the contemplation of the sacred liturgy. In fact, the longest question of the Summa theologiae is about the meaning of the Old Law’s liturgical rituals (1-2.102). The question of the Summa with the most articles is the question on prayer (2-2.83). And no article in the Summa theologiae has more objections than the one about what can go wrong in the celebration of Holy Mass (3.83.5). If we are to renew our liturgical formation through the “rediscovery of a theological understanding of the liturgy,” therefore, as the Holy Father has called for in Desiderio Desideravi, then it is important for us to include the contribution of the theologian par excellence, the Common Doctor of Holy Church, St. Thomas Aquinas.

One place we should look is to St. Thomas’ mystagogy of the Mass—that is, his theological teaching on the meaning and purpose of its various rites, for the sake of helping his fellow Christians better understand the mysteries they celebrate. Unlike his teacher St. Albert the Great and many of their contemporaries, St. Thomas never wrote a stand-alone commentary on the Eucharistic liturgy. Nevertheless, and unbeknownst to many, St. Thomas did write his own expositio Missae or “explanation of the Mass.” In fact, he wrote two: one in his earliest major work, the Sentences commentary (4.8.ex); the other in his latest, the Summa theologiae (3.83.4). Both of these treatments include a division of the liturgy into its essential parts, as well as a thorough study of the words that comprise each part—that is, “the things said around this sacrament,” the many words of the whole Mass that surround the few words of institution. For although only that barest form is necessary for the mere being of the sacrament, all the words of the rite are necessary for its well-being. “Because in this sacrament the whole mystery of our salvation is embraced,” he says, “thus it is carried out with greater solemnity.” Moreover, like every sacrament, the Mass signifies “in a twofold way, namely by words and by deeds.” Thus both in the Sentences (4.12.ex) and in the Summa (3.83.5), Thomas supplements his commentary on the words of the Mass with a further reflection on its actions and gestures—which, he assures us, “are not ridiculous gesticulations, for they are done to represent something.”

It is not a mistake that the liturgy is mysterious: It unveils precisely by veiling.

St. Thomas takes the liturgical details of the Mass tremendously seriously. So great is his respect for these sacred rites that he believes that any celebrant who deviates from them should be threatened with the loss of his priestly office—to say nothing of the grave spiritual danger in which such a minister places himself. In the sed contra of the abovementioned article 4, in defense of the words of the Mass, Thomas quotes a text from Gratian as saying, “James the brother of the Lord according to the flesh, and Basil the bishop of Caesarea, edited the celebration of the Mass”—“from whose authority,” Thomas continues, “it is clear that each and every thing [said] around this [sacrament] is said fittingly.” Then in the sed contra of article 5, against various objections that the actions of the Mass are unfitting, he simply says, “But to the contrary is the custom of the Church, who cannot err, as she is instructed by the Holy Spirit.” Thus St. Thomas understands the words and actions of the Eucharistic liturgy to have been handed down on the authority of the apostles, the Fathers, the Church, and, indeed, God himself.

St. Thomas had learned from Aristotle that “it belongs to the wise man to order,” and he applies that lesson whenever he grapples with a new text. For Thomas, it is not enough to understand the various contents of a text, its matter, if one does not also understand the order of those contents, its form. The missal is no exception—although granted, when treating the Mass, St. Thomas is not primarily commenting on a book, but rather on the celebration itself, in word and in deed, which uses that book. He begins his division of the Mass from the Dionysian exitusreditus principle (that is, the principle that all things flow forth from our good God and are ultimately drawn back to him): “Because every operation of ours is begun by God, so it ought to be terminated in him, coming full circle. Thus the office of the Mass begins with prayer, and is terminated in thanksgiving.” St. Thomas therefore divides the Mass into three main sections, the first of which corresponds to the aforementioned prayer or exitus, and the third to that thanksgiving or reditus, with everything in between making up the second, and by far the largest, part of the circle. Thomas subdivides each of these many times over, before offering further divisions based upon the speakers at the Mass: priest, ministers, and choir—as well as based upon the principal significations of its symbols: the representation of the Passion, the disposition of the Church, and the devotion and reverence due to this sacrament.

“Because in this sacrament the whole mystery of our salvation is embraced . . . thus it is carried out with greater solemnity.”

After then treating each moment of the Mass in painstaking theological detail (which you can read in my book), by way of epilogue or grand finale, St. Thomas concludes with a reflection on the different languages used in the Mass. Some of its words are in Greek: for example, “Kyrie eleison, ‘Lord have mercy.’” Others are in Hebrew: “such as Alleluia, ‘Praise God’; Sabaoth, ‘hosts’; Hosanna, ‘Save, I beg’; Amen, ‘Truly’ or ‘Let it be done.’” Most of the rest of the Mass as he knows it, obviously, is in Latin. St. Thomas teaches that the combination of these three languages in the liturgy (Greek, Hebrew, and Latin) is beautifully fitting. For the Mass is a representation of Christ’s Passion—and of course, “the titulus of the cross of Christ was written in these three languages.”

In his Scholastic mystagogy, St. Thomas Aquinas reads the Mass in a way analogous to how he reads the Sacred Scriptures: First he considers the rite itself, then the deeper significance hiding within it, which opens up for those who seek with faith. Like all the best things, the Mass is hard for us to understand. Its texts are hieratic and exotic and often inaudible, its movements hierarchical and ritualized and often invisible. In recent years, we have seen many liturgists try to solve for this difficulty by calling for the Mass to be ever more simplified, made transparent and plain, and translated down into an everyday idiom. But if the mystical meaning of the Mass is something like the spiritual sense of Sacred Scripture, then on St. Thomas Aquinas’ principles, too much popularization would be a terrible mistake. It would be like replacing the inspired Word of God—which is often similarly difficult and obscure—with a children’s picture Bible. Whereas for St. Thomas, the Scriptures are difficult by design, not only because the richness of the form should fit the richness of the content, but also because an easier text would not hold our attention anyway. St. Thomas’s expositiones Missae suggest that the same logic applies to the rites of the Mass. It is not a mistake that the liturgy is mysterious: It unveils precisely by veiling. After the example of St. Thomas, the right response to the difficulty of understanding the Mass is not to dispel the difficulty, to dispel the mystery, but to contemplate it. Perhaps, therefore, the best way to read St. Thomas Aquinas’s theology of the rites of the Mass, is as a liturgical lectio divina.