There are lots of attempts to debunk the idea that some music is sacred (in style, not just text) and some is not. Mostly, this issue is over-thought to the point of confusion, and lots of conventional wisdom, even when it is true, is the source of the confusion.
For example, I'm sure the following has happened to you. You point to your favorite hymn, and some member of the bourgeoisie pipes up to point out that it is actually an old German drinking song with the words changed. Suddenly your mind races away from a holy place to a tavern with beer flowing and maids serving.
On a more sophisticated level, you point to your favorite Mass by Josquin and a musicologist points out that it is based on a secular chanson, which is a secular poem. This game can go on to the point that a serious musicologist can argue that there is no such distinct category called sacred music and that all music typically called that has secular origins. If we take this far enough, we plunge into a kind of relativism and artistic agnosticism: perhaps the whole distinction is a fabrication.
Note that the causal line promoted by this agnosticism rarely runs in the reverse, such that secular music is actually based on sacred music, though it would make sense if true. The great musical innovations from the year 1000 forward, and the great efforts to preserve and notate music in the previous millennium, centered on the chants of the Church. Sacred music surely provided a model here. But no, instead, we are constantly encouraged to see the sacred world as a parasite on the secular.
I'm not a musicologist and cannot comment on the relative weight of the borrowings both ways, but I do know from my own studies of technology and text that originality is one of the most over-valued notions in human history. Most all things come from somewhere and build toward something. There is no strict line separating sacred and secular in this sense anymore than there is a strict line between any innovations in human history.
The way influence and emulation interact in the sweep of history is elusive to even the most erudite scholars. It also makes sense that many things secular would have its own sacred counterpart and visa versa. This is true for vestments, for example, and architecture as well. It is also true of music.
Nonetheless, none of these caveats erase the distinction between sacred and secular music, or blot our the reality that this distinction has persisted from the earliest to our age. Think of the early Christians, who were surrounded by metered Greek secular music with rhymes. They rejected it all, in favor unmetered music in prose: prayer than is sung rather than merely secular music with religious texts. Standards loosened as the centuries progressed of course, but the essential difference remained.
Toward convincing you that there is something called sacred music, let me draw your attention to the music of Franz Lizst (1811-1886). Like most people, my introduction to his work came with the ever-popular Hungarian Rhapsody and then his famous melody Liebesträume.
I've enjoyed CDs of his piano works enormously. They are thrilling and delightfully over-the-top. And I've read accounts of his life and fame as the first real performer "rock star" in history: the crazed personality cult surrounding his talents, the mob-like behavior that would greet him as he walked the streets, the public collecting of relics from his person. It is all quite hilarious.
Then an interesting thing happened last year at the Colloquium. We needed one final piece to sing because the piece we picked was too much to take on so late in the week. One conductor suggested Ave Maris Stella by Franz Liszt. Until that moment, I'm not even sure that I knew that he wrote sacred works. I'm not sure what I was expecting, but when the downbeat came in the first rehearsal, I came to know a Liszt that I had never known. The music was deferential to tradition. It was holy. It was humble. It was all about the theme of celebrating Mary.
Nowhere in this piece was the Liszt personality cult I had enjoyed reading about. There was no sign of the dashing concertizer who smashed pianos and broke strings with explosive performances. This hardly seemed like the Liszt I knew at all. And yet, there were interesting harmonies, comforting but intriguing modulations of lines and chords, and perfect drama I had detected from other works but poured into spiritual ends. Liszt himself had vanished into this piece but for one thing: his talent. But his talent was stripped of egoism. Instead, his talent was being given to God.
It turns out that Liszt was a serious Catholic, and he knew the Breviary well. Following several family tragedies, he even moved to Rome to devote himself to prayer and good works. In total, he had actually composed more than 60 choral pieces on Catholic themes, each bearing the marks of the Ave Maris Stella. The difference with his secular works could not be more striking. No one can miss this difference or rationalize it away under the idea that there really is no difference.
Liszt understand the particular demands of the sacred. It has a particular style, and special spirit, a different intention. In the end, the music sounds like Church because it strives to be holy, beautiful, and universal. It takes the listener outside the world of the composer and performers and shows them something beyond time and beyond talent. The same could be said of innumerable other composers in history. They understood the difference because the difference is real and substantial.
All the pettifoggery in the world cannot erase the difference between secular and sacred art. If one borrows from the other for the general purpose of artistic improvement, that is much welcome. If secular music becomes more entertaining due to sacred influence, that's fine. If sacred music becomes all the more inspiring due to secular influence, that is fine too. But noticing mutual influence is not the same as demonstrating identity.
A major problem today is that much of the music that is performed in our Churches does not have the marks of sacred music. This is not because such marks are myths; it is beause the composes, publishers, and singers have lost their capacity to imagine that holiness in art is possible at all. There are many remedies, but listening to how Liszt managed to reach toward Heaven is a good beginning toward new understanding.
The Ave Maris as performed at the colloquium is on the colloquium CD . You can also listen to the entire Ave Maris Stella by Liszt.