It is impossible to get behind the Christian uses, in which from the earliest times it was employed as the equivalent of the Latin sacer and sanctus.
It so happens that the supremely important parts of the Mass are those which have the smallest number of words, namely the Kyrie, important as being the opening prayer; the Sanctus and Benedictus, embodying the central acts and ideas of the service; and the Agnus Dei, the prayer with which it concludes.
Accordingly, Palestrina and his great contemporaries and predecessors treated the Gloria and Credo in a style midway in polyphonic organization and rhythmic breadth between that of the elaborate motet (adopted in the Sanctus) and the homophonic reciting style of the Litany.
It is almost impossible, without asceticism of a radically inartistic kind, to treat with the resources of instrumental music and free harmony such passages as that from the Crucifixus to the Resurrexit, without an emotional contrast which inevitably throws any natural treatment of the Sanctus into the background, and makes the A gnus Dei an inadequate conclusion to the musical scheme.
The primitive notion that perhaps comes nearest to our " sacred," whilst it immediately underlies the meanings of the Latin sacer and sanctus, is that of a taboo, a Polynesian term for which equivalents can be quoted from most savage vocabularies.