I will praise thee, O lord, with my whole heart: for thou hast heard the words of my mouth. I will sing praise to thee in the sight of his angels: I will worship towards thy holy temple, and I will give glory to thy name. For thy mercy, and for thy truth: for thou hast magnified thy holy name above all. In what day soever I shall call upon thee, hear me: thou shall multiply strength in my soul. May all the kings of the earth give glory to thee: for they have heard all the words of thy mouth. And let them sing in the ways of the Lord: for great is the glory of the Lord.
~Psalm 137: 1-5
"The musical tradition of the universal Church is a treasure of inestimable value, greater even than that of any other art. The main reason for this pre-eminence is that, as a combination of sacred music and words, it forms a necessary or integral part of solemn liturgy." The composition and singing of inspired psalms, often accompanied by musical instruments, were already closely linked to the liturgical celebrations of the Old Covenant. The Church continues and develops this tradition: "Address . . . one another in psalms and hymns and spiritual songs, singing and making melody to the Lord with all your heart." "He who sings prays twice."
Song and music fulfill their function as signs in a manner all the more significant when they are "more closely connected . . . with the liturgical action," according to three principal criteria: beauty expressive of prayer, the unanimous participation of the assembly at the designated moments, and the solemn character of the celebration. In this way they participate in the purpose of the liturgical words and actions: the glory of God and the sanctification of the faithful:
How I wept, deeply moved by your hymns, songs, and the voices that echoed through your Church! What emotion I experienced in them! Those sounds flowed into my ears distilling the truth in my heart. A feeling of devotion surged within me, and tears streamed down my face - tears that did me good.
The harmony of signs (song, music, words, and actions) is all the more expressive and fruitful when expressed in the cultural richness of the People of God who celebrate. Hence "religious singing by the faithful is to be intelligently fostered so that in devotions and sacred exercises as well as in liturgical services," in conformity with the Church's norms, "the voices of the faithful may be heard." But "the texts intended to be sung must always be in conformity with Catholic doctrine. Indeed they should be drawn chiefly from the Sacred Scripture and from liturgical sources."