Does anyone know when the Church started the Responsorial Psalm format, and why we do that instead of reading the entire psalm at Mass like we do with the Readings and the Gospel?
For the Roman rite at least, the Responsorial Psalm in its present format on the one hand is a recent thing - it was introduced when the order of the Mass was revised after Vatican II. But on the other hand, it’s a (partial) revival of ancient practice.
Back in the old days (specifically, I have in mind the time after the 4th century but just before the Middle Ages) in Rome, the Psalms was a main (but of course not the only) source for hymns throughout the whole liturgy: a psalm was often sung during the entrance procession (the
Introit), usually after the Epistle - what would now be the first reading (the
Gradual*), sometimes during the Offertory, and sometimes during Communion. (During Lent, a
Tract replaces the Alleluia that was sung after the Gradual; psalms were also often used for the Tract.) Much of the same thing is done in many other places (there were originally many local liturgies; Rome was just one of them).
Out of these, the Gradual is the oldest chant: ever since the early days of the Church there seems to have been a custom of reciting a psalm in between the readings. The other chants, on the other hand, were introduced later to fill up the silence during the entrance procession or communion (when large churches meant a long procession or a lot of people attending the Mass). The Gradual, Alleluia, and (for a time) the Offertory were
responsorial chants (a solo cantor sings the verses while the choir answers with a refrain called a
respond), while the Introit and the Communion chants were
antiphonal (two choirs alternate: one choir sings the verses, the other sings a refrain called the
antiphon). (The Tract is
direct psalmody: the psalm was sung without refrain in alternation by two halves of the choir.)
But by the Middle Ages, what happened was that these chants were drastically shortened, so that only a verse or two was really sung or recited. For example, the Introit for the Mass of Easter Sunday in the Extraordinary Form/Tridentine Mass is Psalm 138/139:5-6 (antiphon) and Psalm 138/139:1-2:
I arose, and I am still with you, alleluia; you lay your hand upon me, alleluia; your knowledge is become wonderful, alleluia, alleluia.
O Lord, you have searched me and known me; you know my sitting down and my rising up.
Glory be to the Father…
As it was in the beginning…
I arose, and am still with you…
Originally, this would have been a fuller psalm:
I arose, and I am still with you, alleluia; you have laid your hand upon me, alleluia; your knowledge is become wonderful to me; it is high and I cannot reach to it, alleluia, alleluia.
O Lord, you have searched me and known me; you know my sitting down and my rising up.
I arose, and am still with you…
You have understood my thoughts from afar; my path and my fate, you have searched out.
I arose, and am still with you…
(And so on until the psalm finishes or the choir is ordered to finish)
Glory be to the Father…
As it was in the beginning…
I arose, and am still with you…
- While the responsorial psalm now occupies the place of the Gradual, and while it can be seen as a revival of the ancient practice, they’re technically not exactly the same, in that going by the liturgical books they’re really different options: in a Mass, the Gradual chant (which retains the format of the Tridentine Mass instead of the modern one, printed in a book called the Roman Gradual) can be used in place of the responsorial psalm (the one printed in the Lectionary), though in reality this is a little-used option. The issue is a bit complicated.
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