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In the Dogmatic Constitution on Divine Revelation Dei Verbum, the Second Vatican Council teaches us about the relationship between Scripture and Tradition:
There exists a close connection and communication between Sacred Tradition and Sacred Scripture. For both of them, flowing from the same divine wellspring, in a certain way merge into a unity and tend toward the same end. For Sacred Scripture is the word of God inasmuch as it is consigned to writing under the inspiration of the divine Spirit, while Sacred Tradition takes the word of God entrusted by Christ the Lord and the Holy Spirit to the Apostles, and hands it on to their successors in its full purity, so that led by the light of the Spirit of truth, they may in proclaiming it preserve this word of God faithfully, explain it, and make it more widely known. Consequently, it is not from Sacred Scripture alone that the Church draws her certainty about everything which has been revealed. Therefore both Sacred Tradition and Sacred Scripture are to be accepted and venerated with the same sense of loyalty and reverence. Sacred Tradition and Sacred Scripture form one sacred deposit of the word of God, committed to the Church.When we speak of Scripture, it’s clear (or clear enough) what we are referring to: the contents of the Bible, the canon of writings established by the Church. But when we speak of Tradition, what exactly are we referring to? Where, to put it more concretely, do we meet up with or run into Tradition? When do we find ourselves in its presence? How do we know we’re dealing with “Sacred Tradition”—which the Council says is part of the very word of God!—and not with mere “traditions of men” that may or may not be from Christ the Lord?
Dom Mark Kirby, Prior of Our Lady of the Cenacle Monastery in Ireland, speaks of “the age-old law that grounds and shapes both Catholic doctrine and the Catholic moral life: Lex orandi, lex credendi, lex vivendi.” This is a pithy way of saying that “the law of praying” (how we pray) shapes “the law of believing” (what we believe), which in turn informs “the law of living” (how we actually lead our lives).
The lex orandi is the enactment of the sacred liturgy; it is composed not only of texts, but also of the whole complexus of sacred signs, gestures, and rites by which, through the mediating priesthood of Jesus Christ, men are sanctified and God is glorified. The sacred liturgy itself—being the Holy Sacrifice of the Mass, the other sacraments, the Divine Office, and the various rites and sacramentals found in the Church’s official liturgical books—is the Church’s theologia prima. ... The Church’s primary theology is not something invented by learned men; it is found in the givenness of the liturgy, the primary organ of the Church’s authentic tradition.This conclusion is echoed in the eloquent statement of Fr. Louis Bouyer:
It is in the celebration of the liturgy’s mysteries, and in all of the new, mystical, and communal life flowing from it, that the Church maintains in unity the perpetual and perpetually living consciousness of the immutable deposit of faith entrusted to her.More succinctly still, Pope Pius XI declares: “The liturgy is the principal organ of the Church’s ordinary Magisterium.” A contemporary anonymous writer draws out the implications of this special status:
The liturgy is the primary font or source of our knowledge of revelation. … [I]t is the ordinary, normal context wherein Christian worshipers encounter the divine realities in such a way as to participate in them contemplatively and prayerfully. Encyclicals and councils serve the primarily didactic purpose of informing the intellect of the individual truths of faith—a necessary thing in the Christian life. But the liturgy does this and more. The liturgy is where this formation of the intellect bears its fruit in the living out of faith. The liturgy is faith in practice. It is where Christians receive revelation, believe in it, and act upon that belief by directly worshiping their Creator. … The liturgy, too, is a medium through which revelation is communicated. Indeed, as stated before, it is the definitive and primary context wherein this communication and reception of revelation occurs for Christians, precisely because it is the central act of Christian worship. Worship is the principal act of religion; all other acts are vain unless directed to the act of worship.Because of this intimate connection between how we pray, what we believe, and how we lead our lives, the saints have always shown a burning love for the liturgy and everything connected with it. Its phrases and gestures fill their imaginations. They feel a sense of awe, reverence, and humility before this holy inheritance and they counsel caution in tampering with it. A learned Benedictine of our time, Dom Bernard Capelle (1884–1961), when asked by a Vatican commission to share his opinion about liturgical reform, wrote in 1949:
Nothing is to be changed unless it is a case of indispensable necessity. This rule is most wise, for the Liturgy is truly a sacred testament and monument—not so much written but living—of Tradition, to be reckoned with as a locus of theology and a most pure font of piety and of the Christian spirit.Here, too, we can start to see connections between what I argued concerning the Book of Revelation (the cosmic centrality of worship and the heavenly liturgy of the Church Triumphant as paradigmatic for the Church Militant on earth) and what one learns from Romano Guardini’s Sacred Signs on the language of symbols, through which we come to understand and relate with God, and by which we express what is most inward and most exalted in ourselves.
Tying together the preceding points, Dom Daniel Augustine Oppenheimer shows the ethical and spiritual demands placed on the believer by the sacred liturgy:
Before all else … the primary antecedent is humility before the source itself. Already the ascetic principle of faith is operative, understanding that the liturgical traditio is not “some old piece of cloth,” to use a famous phrase of Cardinal Ottaviani, open to free flight or arbitrary incisions and repiecing. The texts, gestures, signs, symbols, music, the full panoply of the liturgical culture, all have together an internal cohesion, sense, depth, and character. The thing itself deserves reverence because it is holy and the principal font of revelation.
LEX CREDENDI & LEX VIVENDI
Coming now to the second and third members of our “age-old law,” Dom Mark writes:
The lex credendi is the articulation of what is already given, contemplated, and celebrated in the lex orandi. The Church’s doctrine emerges in all its shining purity—in the veritatis splendor—from the wellspring of her liturgy. Catholic doctrine, the Church’s theologia secunda, is the fruit of her liturgical experience. … The lex vivendi is the Catholic moral life, a life quickened by the theological virtues, a life in obedience to the divine commandments, characterized by the cardinal virtues, illumined by the Beatitudes, enriched by the Seven Gifts of the Holy Spirit, and displayed in the Holy Spirit’s Twelve Fruits. The lex vivendi pertains to all that teaches men to live rightly, to every ethical and social question, and to the pursuit of that holiness that already we contemplate in the saints set before us by the Church.The order of the three elements is by no means accidental: as we have seen, the liturgy delivers to us the faith we profess, or put differently, we profess our faith in and through the liturgy. Divine worship, as handed down from the apostles and their successors, comes first, fills our minds and hearts, and shows us the way; the theological articulation and explication of the faith comes second, as an internalization of and reflection upon what we are doing when we celebrate the sacred mysteries. Once we have turned in prayer to the living God, who is Alpha and Omega, the First and the Last, giving Him the primacy that is His due (the lex orandi)—and once we have received the truth from His lips and His hands, giving truth the primacy in our souls (the lex credendi)—then we will have our “marching orders” for life in the world, the fulfillment of righteousness in virtuous love of ourselves and of our neighbor (the lex vivendi). Dom Mark captures this order nicely:
The restoration of Catholic doctrine in all its beauty and richness, and the consequent reclaiming of Catholic discipline as something both healing and life-giving, will begin with the restoration of the sacred liturgy.Another pseudonymous author offers us a powerful meditation on the super-realism of the liturgy, which, by really containing what it represents, puts us in direct, immediate contact with the ultimate realities:
Liturgy does not merely teach belief and transmit grace. It revives and renews the sacred mysteries of Christ in time for the faithful. In doing so, one encounters Christ, the angels and saints, and glimpses the greater spiritual reality of the Lord while remaining on earth, blurring the lines which separate the eternal and the temporal. One leaves the liturgy and the “mystical supper” of Christ not only having learned what to believe, but also how to believe when he returns to the world outside the temple. … How does one orient one’s self to God and away from sin? How does one see the world and God as He wishes? The liturgy shows us this, in conjunction with being the setting for the Sacraments, wherein the Holy Spirit acts and makes the work of Christ immediately accessible to the believer. … The purpose of the liturgy, especially during the great periods of the year, is to unite the faithful to God so that they might know Him and save their souls. He gathers them to Himself and to His new Jerusalem, the Church, and to His Body, again, the Church.
 Dei Verbum, n. 9.
 All the quotations of Dom Mark are from his article “Liturgy, Doctrine, and Discipline: The Right Order." See also Joyce Little's article "Lex Orandi, Lex Credendi: Many Young Catholics Find Liturgy Incomprehensible and Irrelevant. Is It?"
 Cited in George Cardinal Pell's "The Translation of Liturgical Texts" (and by many other authors).
 The Maestro, "Liturgy, Revelation, and Tradition."
 Cited in Paweł Milcarek's excellent article “Balance Instead of Harmony."
 Dom Daniel Augustine Oppenheimer, "Asceticism and Tradition."
 The Rad Trad, "Liturgy & Tradition: Sensus Fidelium."