Here is an excerpt from an outstanding article appearing in The Catholic Sun written by Bishop Thomas Olmsted on the basics of sacred music. He addresses an important yet often overlooked distinction between sacred music and religious music.
What is Sacred Music?
Sacred music is, in the narrowest sense, that music created to support, elevate, and better express the words and actions of the sacred liturgy. The Council praises it as music “closely connected…with the liturgical action” (Sacrosanctum Concilium, 112) …
Sacred music is distinct from the broader category of what we may call “religious” music, that which aids and supports Christian faith but is not primarily a part of the sacred liturgy. “Religious” music includes various devotional music, such as much popular hymnody, “praise and worship” music, as well as a host of other musical forms.
… but sacred music expresses something utterly unique: the saving and sacrificial prayer of Christ and the Church in the liturgy.
This is the first article in a series by Bishop Olmsted on the subject, so be sure to follow along.