Following the practice of the Jewish synagogue, the community traditionally replies to a scriptural reading by singing a psalm or biblical canticle. In Rome a sub-deacon or cantor mounted the ambo and chanted the text, which came to be called the gradual, a name taken from the steps, i.e. gradus, of the ambo. Today it is called the responsorial psalm, a designation expressing the structural nature of the text and the manner in which it is used as a response to the proclaimed Word of God.
Although some call attention to the meditative quality of the psalm, reflection is not its primary function. Rather, the psalm serves as the people's response to the reading just proclaimed. The scriptural message is to reverberate in the assembly whose members together acknowledge and respond to the word of God by using the word of God.
The selection of the responsorial psalms found in the Lectionary was done with the utmost care. Certain general principles were followed. Thus a psalm is used for a response if the Scriptures for the day quote the psalm, if a literary reference is made to the psalm in the first reading, if the psalm more clearly illustrates what is proclaimed in the reading. Additionally, psalms having a connection with a particular liturgical season are used during that season, e.g. the penitential psalms during Lent. Furthermore, a selection of psalms appropriate to each season of the year is also given.
The Order of Mass considers it normative that the responsorial psalm be sung. The reason is threefold: the genre of the psalms as lyrical compositions calls for singing; the psalm is a response to the spoken word and ritual structure does not customarily respond to speech with more speech; this is the only time in the liturgy when a psalm is used for its own sake and not to accompany a ritual action. Every effort, therefore, is to be made to sing the psalm response.
The person leading the assembly in the singing of the responsorial psalm is referred to as the cantor or the psalmist. The psalm should be sung at the ambo, since the psalm is part of the Word of God and the ambo is the place from which that Word is proclaimed.
The resonsorial psalm is the assembly's acclamation of the proclamation of God's Word in our midst. That is how we worship: proclamation followed by acclamation.
[Source: Sunday Bulletin, St. Paul Roman Catholic Cathedral, Saskatoon, SK, Canada; April 27, 2008]