"Sacred" itself is a term that is very seldom used. I never hear it. I think most churches would hope that they were not sacred, because it sounds like something that would turn people off, and hurt church growth. Churches do not want to be sacred, but accessible and tolerant. Sacred seems exclusive and boring and dusty, even proud.
On the other hand, if I asked church members if they believed in "sacred music," they would say, "yes," but they wouldn't know what it was. They think it is sacred because it is being used in the church, just because it is church music. That alone makes it sacred. Or that it came from a Christian music publisher, to be used by churches and religious organizations. To these same people, the words are what make the music sacred. If it has Christian words, it is sacred music. What I'm saying is that even of those who would support sacred music, they don't know what it is.
You may have heard the question, "Is nothing sacred anymore?" The pop performer, Meat Loaf, has a song, Is Nothing Sacred Anymore? I wouldn't have known that if I had not googled that question, wondering what would come up. That should tip you off to the confusion. The irony here is a person named Meat Loaf singing about nothing being sacred, in case you didn't know. The concept of "sacred" isn't sacred. The very definition of sacred has been profaned when no one blinks at Meat Loaf.
In the song, Meat Loaf sings about "love." He sings about love in a profane way, in a way that love itself is no longer sacred. Love is redefined and sentimentalized to something that isn't even love. In other words, when Meat Loaf sings about it, love isn't sacred anymore. Love is a biblical word. It shows up in our culture, because it started in God's Word. A sacred understanding of love is a biblical view of love, but I digress.
You will not find the English terminology "sacred music" used until the seventeenth century, and then not much in printed literature. It increases in the eighteenth century, but not much, and you read its usage in 1798 in a book entitled, Melody, the Soul of Music (p. 48):
Even sacred music is not exempted; particularly when accompanied with instruments. Frivolous and airy passages sometimes mingle with such strains as should never deviate from the solemnity which suitable ideas of the Being to whom they are addressed ought always inspire. Even in Hallelujahs and expressions of devout gladness, we should, in the language of Scripture, "join trembling with out mirth." Owing to inattention in this respect, some pieces of sacred music, intended for that kind of expression, seem better calculated to promote convivial hilarity.
This reminded me of something Thomas Ross posted here awhile back on this very subject, entitled Reverence and Solemnity: Essential Aspects of Biblical Worship. These characteristics distinguished sacred music from other music. We read from George Horne (1784), The Antiquity, Use and Excellence of Church Music (p. 11):
For there is no doubt but that the heart may be weaned from everything base and mean, and elevated to every thing that is excellent and praiseworthy, by sacred music. The evil spirit may still be dispossessed, and the good spirit invited and obtained, by the harp of the son of Jesse.
Sacred music must be music, of course, but most of all, it must be sacred. A sermon in 1719, entitled The Holiness of Christian Churches, by Thomas Mangey, gives a good understanding of "sacred":
Whatsoever is dedicated to God, is by that solemn Act of Dedication, made so entirely his, that the Application of it to civil Uses, tho' jointly with Sacred, is one Degree of Sacrilege.
They thought that something that was sacred was just sacred. This is also the understanding in the Baptist New Hampshire Confession of Faith (1833):
That the first day of the week is the Lord's-Day, . . . and is to be kept sacred to religious purposes, by abstaining from all secular labor and [sinful] recreations; by the devout observance of all means of grace, both private and public; and by preparation for rest which remaineth for the people of God.
If something is kept sacred, it is kept "holy." As "sacred" relates to God, the music, if it is sacred, should be characterized by those perfections of God's attributes. The music should be scriptural in every way and according to the qualities that accord with God. To understand sacred, it helps to understand the antonym, which is common or profane.
Sacred music should not be in character like the world's music. There is still sacred music. People don't like it. It's all that God accepts, but it's not what churches want to use for worship, because people have a different taste of music. The people have become common and profane and they offer what they like to God. He doesn't accept it, but it tells you about the people.
God wants a distinction between what is common or profane and what is holy or sacred. The sacred should conform to God -- His character, His nature, His Person. I repeat: that music does exist and can be used. Churches have stopped using it because the people don't like it. In not using it, they are saying that they don't like God. This is idolatry. How?
When someone gives God what he likes, but God doesn't like it, he is worshiping himself. When someone offers something to God that God doesn't accept, He is worshiping something of a different nature than God, so it is a different God. Worshiping the wrong way leads to worshiping the wrong god. Either way though, God isn't worshiped.