It may seem surprising, but only sixty or so years ago there was virtually no singing during the liturgy. In those days, prior to the Second Vatican Council, the Mass was celebrated in two ways: the High Mass and the Low Mass. The High Mass had singing, with priest, deacon and choir having their assigned parts. These were complex liturgies, and were rare in the average parish because of the lack of deacons, subdeacons and competent choirs. The members of the congregation remained silent throughout.
The Low Mass was the liturgy that most Catholics encountered on Sundays and weekdays. There was rarely any singing by anyone. Even if congregational singing were allowed, it would have been in the form of Latin chants. Hymns in English (or the language of the people) were forbidden.
All of this would change in 1963 with the publication of the Constitution on the Sacred Liturgy. In this foundational document for liturgical renewal, the Church solemnly taught that “A liturgical service takes on a nobler aspect when the rites are celebrated with singing, the sacred ministers take their part in them, and the faithful actively participate… Bishops and pastors must be at pains to ensure that whenever a liturgical service is to be celebrated with song, the whole assembly of the faithful is enabled, to contribute the active participation that belongs to them.”
The ancient Latin Gregorian chants were still given “pride of place” in the liturgy, but in practice chants and hymns in the language of the people have become the norm throughout the world. The Constitution encouraged new musical compositions, not just suited to large choirs, but that would engage everyone in the assembly. In addition, the texts meant to be sung were to “always be consistent with Catholic teaching; indeed, they should be drawn chiefly from holy Scripture and liturgical sources.”
The most important parts to be sung in the liturgy are the acclamations: the Alleluia before the gospel, the Holy, Holy, the Memorial acclamation, the Great Amen, and For the Kingdom, the power, and the glory... after the Lord’s Prayer. Next in priority to be sung are the two processional chants, the entrance song and the Communion song. In most cases these are popular hymns taken from the hymnal. The responsorial psalm also is best sung. Other parts may be sung as well, but priority should always be given to the acclamations. For example, a liturgy that includes an opening and closing hymn, and perhaps a hymn at the preparation of the gifts and at Communion time, but ignores the Alleluia and the Great Amen, is musically deficient.
The popular songs in our hymnals are always a source of divergent opinion, as are all forms of art. Some new hymns will someday be considered traditional, while most will not make the list of classic hymns. We also have a great treasury of hymns from other faith traditions, from the Protestant tradition in particular. Our hymnals normally include hymns by Protestant composers. It is interesting to note that the Vatican’s Directory for the Application of Principles and Norms on Ecumenism, published in 1993, even recommends that both Catholic and Protestant hymnals include a collection of hymns that might be sung when Catholics and Protestants gather to worship together.