When thinking and writing about music appropriate for liturgical worship I have always gone back to what the late Church musician Msgr. Richard Schuler correctly asserted: since sacred liturgical music is NOT an add-on in worship, since it is actually an integrating part (pars integrans) of liturgical worship, since it is prayer, liturgical music must be both sacred and also art.
The texts must be sacred texts. The idiom must be a sacred idiom, or at least not opposed to the sacred. The music must be good, that is, well-composed, of high artistic value. It must be performed well.
Music for liturgical worship must be sacred and it must be art.
If the music chosen does not fulfill those criteria, it does not belong in the Mass.
The music itself becomes prayer within the liturgical setting. People pray also by listening to true sacred liturgical music. It is prayer.
We cannot ever go wrong when we stick to the texts actually assigned by the Church for each Mass or office. We cannot ever go wrong when we use Gregorian chant and polyphony and the pipe organ, as the Fathers of the Second Vatican Council established as having the first place among all genres of music for sacred worship.
I have mentioned in the past Msgr. Valentin Miserachs-Grau, the head of the PIMS, the Pontificia Istituto di Musica Sacra in Rome.
Miserachs-Grau has given a good address, which NLM has reproduced in its entirety. Here, however, is the meat of the matter, with my emphases and comments.
This one is a sore point: the rampant wave of false and truly dreadful liturgical music in our churches. Nevertheless, the will of the Church clearly appears in the words of the Holy Father I have just mentioned. [... he cites documents... ] Now we must wonder: if the will of the Church has been clearly declared also in our times, how is it possible that the musical praxis in our churches distances itself in so evident a way from the same doctrine? [NB: "doctrine".]
We must consider several problems at the root of this question, for instance the problem of repertoire. We have hinted at a double aspect: the risk of shutting oneself in a closed circle that would wish to essay new compositions considered as being of high quality in Liturgy. We must say that the evolution of musical language towards uncertain horizons makes the breach between “serious” music and popular sensitivity to become more and more profound. Liturgical music must be “universal”, that is acceptable to any kind of audience. [?! Really. I don't think I can accept that given the degradation of culture and taste these days. Still, well-sung Gregorian chant pretty much fits that description, as does polyphony.] Today it is difficult to find good music composed with this essential characteristic. I do not discuss the artistic value of certain contemporary productions, even sacred, but I think that it would not be opportune to insert them in the Sacred Liturgy. One cannot transform the “oratory” into “laboratory”.
The second aspect of the problem derives from a false interpretation of the conciliar doctrine on Sacred Music. As a matter of fact, the post-conciliar liturgical “renewal”, including the almost total lack of mandatory rules at a high level, has allowed a progressive decay of liturgical music, at the point of becoming, in the most cases, “consumer music” according to the parameters of the most slipshod easy-listening music. [Attention:] This sad practice sometimes determines attitudes of petulant rejection towards genuine Sacred Music, of yesterday and today, maybe composed in a simple manner, but according to the rules of Art. [WHAT?! Art has RULES?!?] Only a change of mentality and a decisive “reforming” will – that I am afraid is far to come – would be able to bring back to our churches the good musical praxis and, together with it, also the conscientiousness of celebrations, that would not lack to entice, through the value of beauty, a large public, particularly young people, currently kept away by the prevailing amateurish practice, falsely popular and wrongly considered – even in good faith – as an effective instrument of approaching. [My own experience of when I was rector of a church in Italy, is that when we used good chant and polyphony, and when we opened the church doors, people came in and stayed.]
Regarding the power of involvement of which the good liturgical music is capable, I would like to add only what is my own personal experience. By a fortunate chance, I am acting after almost forty years, as Kapellmeister at the Roman Basilica of Saint Mary Major, where every Sunday and on feast days the Chapter Mass is celebrated in Latin, and with Gregorian and polyphonic chant accompanied by organ (and by a brass sextet in highest solemnities). I can assure you that the nave and the aisles of the basilica get packed and not rarely there are people that come after the ceremonies to express their gratefulness, moved to tears as they are, especially by the Hymn to the Madonna Salus Populi Romani (Our Lady, Salvation of the Roman People). They often cannot hold back the excitement and arrive to burst out clapping. People are thirsting for good music! It goes directly to the heart and is capable of working even resounding conversions. [Yep. Excellent sacred music, properly understood, played a major role in my own conversion.]
Another compass of good liturgical music –always reminded by the Teaching of the Church– concerns the primacy of the pipe organ. [... he discourses on the organ for a while ...]
In our quick review of the main points underlying a good liturgical musical praxis, we have now arrived to a last but not least question, one that should be firstly considered: the Gregorian chant. It is the official chant of the Roman Church, as the Second Vatican Council reasserts. Its repertoire includes thousands of ancient, less ancient, and even modern pieces. Certainly, we can find the highest charm in the oldest compositions, dated back to the Xth-XIth Centuries. In this case also it has to do about an objective value, since the Gregorian chant represents the synthesis of the European and Mediterranean chant, related to the genuine and authentic popular chant, even that of the remotest regions of the world. It is a deeply human and essential chant that can be traced in its richness and variety of modes, in its rhythmic freedom (always at the service of the word), in the diversity and different degrees of its single pieces, according to the individual to whom the execution is assigned, etc. This is a chant that has found in the Church its most appropriate breeding ground and constitutes a unique treasure of priceless value, even from the merely cultural point of view.
[NB:] Therefore, the rediscovery of Gregorian chant is a sine qua non condition to give back dignity to the liturgical music and not only as a valid repertoire in itself, but also as a source of inspiration for new compositions, as it was the case of the great polyphonists of the Renaissance, who –following the guidelines of the Council of Trent– created the structure bearing their wonderful works departing from the Gregorian subject matter. If we have in Gregorian chant the master path, why not follow it instead of persisting in scouring roads that in the most of cases drive to nowhere? [...]
You can read the rest over there.