Sometimes it's hard to tell the difference.
For example, some people have problems telling the difference between me and my twin sister.
But when it comes to Church music, it's actually not very hard to tell the difference between music that's appropriate for Mass, and music that's not.
If you want to tell the difference between me and my twin, you'll have to spend some time with us.
If you want to tell the difference about Church music, you'll have to spend some time with the Church documents.
The good news is, the Church has clearly laid things out for us.
First of all, a few quotes from the Church documents:
In 2003, Pope John Paul II quoted from Paul VI: "if music - instrumental and vocal - does not possess at the same time the sense of prayer, dignity and beauty, it precludes entry into the sphere of the sacred and the religious"
John Paul II went on to say: With regard to compositions of liturgical music, I make my own the "general rule" that St. Pius X formulated in these words: "The more closely a composition for church approaches in its movement, inspiration, and savour the Gregorian melodic form, the more sacred and liturgical it becomes; and the more out of harmony it is with that supreme model, the less worthy it is of the temple"
Cardinal Arinze reminded us that:
"People don't come to Mass in order to be entertained. They come to Mass to adore God, to thank him, to ask pardon for sins, and to ask for other things that they need."
Or, we look to the 1967 document: MUSICAM SACRAM:
those instruments which are, by common opinion and use, suitable for secular music only, are to be altogether prohibited from every liturgical celebration and from popular devotions.
instruments are suitable for sacred use, or can be adapted to it, that they are in keeping with the dignity of the temple, and truly contribute to the edification of the faithful."
Pope Pius XII wrote in 1955, 41. First of all, the chants and sacred music which are immediately joined with the Church's liturgical worship should be conducive to the lofty end for which they are intended.
In 1958, the Sacred Congregation of Rites stated clearly:
b) The difference between sacred, and secular music must be taken into consideration. Some musical instruments, such as the classic organ, are naturally appropriate for sacred music; But there are some instruments which, by common estimation, are so associated with secular music that they are not at all adaptable for sacred use.
b) they are to be played with such seriousness, and religious devotion that every suggestion of raucous secular music is avoided, and the devotion of the faithful is fostered;
The Second Vatican Council, in its document on the Liturgy, said this:
The Church acknowledges Gregorian chant as specially suited to the Roman Liturgy: therefore, other things being equal, it should be given first place in liturgical services.
Because there are hundreds and hundreds of Sacred Music directives, let's just stop reading quotes, because otherwise we're going to run out of time.
The first thing I noticed when I read these quotes was, it's WRONG when people say, "All types of music are allowed at Mass. As long as the music moves me, it can be used at Mass." This idea is obviously not in accordance with the Church documents.
It's also important to realize that the CAUSE or REASON for this copious Ecclesiastical legislation is very serious. After all, the Mass is the reenactment of the Sacrifice at Calvary. Think about this for a minute. The Mass is the reenactment of the Crucifixion our Lord. Is it any wonder, then, that the Church wants dignified music at Mass?
It always reminds me of the priest or Bishop, who wears precious Vestments at Mass. The celebrant does not wear these ornate vestments to exult himself, or to amuse the congregation. One of the reasons why he wears them is they're the same kind of clothes the ancient Romans wore, serving as a visible reminder of the antiquity of our Faith. The traditional vestments are also a type of "uniform" which unite us to all the Catholics who have gone before us. The beauty of the vestments also edifies the faithful, since God is the Author of all beauty, and it reminds them, in some small way, of the splendor and loftiness of the Holy Sacrifice of the Mass. All of these reasons are the same reasons why we use Sacred music at Mass. And all of these reasons help us to pray.
Following the Church documents we quoted earlier, we have to be careful what kind of music is used at the reenactment of Calvary. Music that may be absolutely wonderful at a party (which is trendy, exciting, emotional, and fun to dance to) is not going to be appropriate at Mass. Dinner music, theater music, parade music, circus music: all these have their place, and they're absolutely perfect for certain occasions. But not Mass.