Why should the Catholic Church use Gregorian chant at Mass? That question can be understood and answered, if we take a bullet train through history:
1300 B.C. to 33 A.D.: The Jewish traditions of music in worship are developed across generations, kingdoms, and temples. The Jewish traditions of chant, worship, and temple are at the core of the culture.
33 A.D. to 800: Musicians in the Catholic Church developed the most sophisticated system of chant in the world based on the Jewish traditions of chant and worship, and also based on the musical systems of Roman chant at the time of the early church.
850 A.D. to 1100: Musicians in the Catholic Church developed multi-voice polyphony and basic music theory which was distributed across Europe. Major treatises like the Musica enchriadis (c.850) and Ad Organum Faciendum (1100) are the texts which were read across Europe, laying out in simple terms how to add multiple voices to chant. All of the sacred music, no matter how many voices are added, is based on Gregorian chant.
1100 A.D. to 1200: Musicians in the Catholic Church developed the rhythmic systems which now all known Western music relies on. (It’s easy for us to think of rhythmic notation as obvious, but it didn’t exist in distributable, standardized written format until the musicians in the Catholic Church developed it.)
1285 A.D. to 1377: Major Catholic composers like Leonin, Perotin, and Guillaume de Machaut based in Notre Dame, France developed new polyphonic forms and compositions which are still performed today. They created new rhythmic notation, rhythmic modes, and all of the breakthroughs in notation that would lead to the flourishing of the Renaissance. Messe de Notre Dame (by Machaut) is still performed today.
1400 A.D. to 1525: The music in the Catholic Mass becames widely expanded through various forms of motets, organum (chant with an added voice), and polyphony.
1525 A.D. to 1600: Josquin des Prez and Giovanni Pierluigi da Palestrina perfect the polyphonic form. Thomas Tallis and William Byrd wrote masterpieces during one of the most prolific and highly regarded periods of music history in England. Tomás Luis de Victoria created some of the finest sacred compositions ever written, and became the most famous musician in Spain during the 16th century. This period in time is considered the height of sacred music in the Renaissance, in both quality and output.
1600 A.D. to 1770: J.S. Bach, Purcell, Handel, Haydn, Buxtehude, Pachelbel, Couperin, Vivaldi, Walther, and Mozart output almost countless Baroque and Classical sacred compositions, while also creating sophisticated and adventurous forms of theory, counterpoint, and composition. Upon the foundation created by these composers, all Western tonal theory rests.
1770 A.D. to 1965: Beethoven, Brahms, Rachmaninoff, Liszt, Messiaen…too many composers to name. The Romantic period of classical music gave rise of course to countless gems in the forms of symphonies, cantatas, concerti, and quartets. At the same time, the vast majority of these composers were drawn to the Catholic forms of music. Tenebrae, Requiem Masses, Mass settings of the ordinary, dramatic choral works based on chant, etc. As music and its endless forms flourished, so did Catholic sacred music. Until…
1965 A.D.: Vatican II finishes up. The council releases its completed documents which are either poorly understood or vague language is used as an excuse for an “anything goes” approach. Catholic sacred music, in the span of just 2 years, becomes dominated with reappropriation. Sacred music is no longer based on the chant of the early church (which was based on Jewish and Roman chant). It was based on what is popular at the time: folk, electric, and rock music. Catholic sacred music largely loses its identity, and its connection to the early Church. The living history of sacred music (that is, sacred music rooted in its incredibly vast tradition being performed regularly) almost completely dies out.
2015 A.D.: Today. 50 years from the conclusion of Vatican II.
So here we are again: why should the Catholic Church use Gregorian chant at Mass?
Simply put, because it is and always has been the musical language of the Catholic Church, and of all Christianity. (This fact has been obscured in the last 50 years, where Catholic sacred music looks and sounds like a poor imitation of what secular culture already does quite well.) Gregorian chant is the music which was sung by the apostles and martyrs of the early church, by St. Augustine, by St. Thomas Aquinas, by St. Francis of Assisi. It is the music which has seen the rise and fall of empires. It is the only music which is authentically Catholic in nature. Also: the Vatican II documents on sacred music tell churches to use Gregorian chant! But that is an unfortunately little known fact, and the instruction was almost uniformly ignored in the 1960s.
And last, but not least: the origin of music reveals its purpose. Music is written to move the emotions and soul of a human being, for some purpose (usually expression of an inner feeling of the composer, in modern times). Some music is written to excite you and make you want to party. Some music is written because the composer is sad, and wants to express it. Some music is written to express disgust with government or the status quo and move you to action. And some music, like Gregorian chant, was sung by Christians who witnessed Jesus’ Resurrection, who were tortured and executed for their faith, who were rounded up and killed until 313 A.D., who built beautiful cathedrals across Europe, and who sung from their hearts to express their faith and share it. Gregorian chant was, and is, sung to glorify God; to meditate on the texts of the Mass; and to lift up the soul in worship.
And it is for all of those reasons together why the Catholic Church should use Gregorian chant at Mass.