Catholic Encyclopedia (1913)/Introit
The Introit (Introitus) of the Mass is the fragment of a psalm with its antiphon sung while the celebrant and ministers enter the church and approach the altar. In all Western rites the Mass began with such a processional psalm since the earliest times of which we have any record. As it was sung by the choir it is not, of course, to be found in sacramentaries; but introits are contained in the first antiphonaries known (the Gregorian Antiphonary at Montpellier, the St. Gall manuscript, that represent a seventh-century tradition, etc.; see Leclercq in "Dict. d'archéologie chrétienne", s. v. "Antiphonaire"). The First Roman Ordo (sixth to seventh century) says that as soon as the candles are lit and everything is ready, the singers come and stand before the altar on either side, "and presently the leader of the choir begins the antiphon for the entrance (antiphona ad introitum)". As soon as the deacons hear his voice they go to the pope, who rises and comes from the sacristy to the altar in procession ("Ordo Rom. I", ed. Atchley, London, 1905, p. 128). There is every reason to suppose that as soon as the Western liturgies were arranged in definite forms, the entrance was always accompanied by the chant of a psalm, which from that circumstance was called at Rome Introitus or Psalmus or Antiphona ad Introitum. The old Gallican Rite called it Antiphona ad Pr legendum; at Milan it is the Ingressa; in the Mozarabic, Carthusian, Dominican, and Carmelite books, Officium. The Introit was a whole psalm sung with the Gloria Patri and Sicut erat verses, preceded and followed by an antiphon in the usual way. No doubt originally it was sung as a solo while the choir repeated a response after each verse (the psalmus responsorius of which we still have an example in the Invitatorium at Matins), then the later way of singing psalms (psalmus antiphonarius) was adopted for the Introit too. The "Liber Pontificalis" ascribes this antiphonal chant at the Introit to Pope Celestine I (422-32): "He ordered that the psalms of David be sung antiphonally [antiphonatim, by two choirs alternately] by all before the Sacrifice, which was not done before; but only the epistle of St. Paul was read and the holy Gospel" (ed. Duchesne, I, Paris, 1886, 230). The text seems even to attribute the use of the Introit-psalm in any form to this pope. Medieval writers take this idea from the "Liber Pontificalis", e. g. Honorius of Autun, "Gemma animæ" (in P. L., CLXXII): "Pope Celestine ordered psalms to be sung at the entrance (ad introitum) of the Mass. Pope Gregory [I] afterwards composed antiphons in modulation for the entrance of the Mass" (I, lxxxvii). Probst thought that Gelasius I (492-96) invented the Introit (Die abendländische Messe vom 5 bis zum 8 Jahrhundert, Münster, 1896, 36). It is perhaps safest to account for our Introit merely as a development of the processional psalm sung during the entrance of the celebrant and his ministers, as psalms were sung in processions from very early times. But it soon began to be curtailed. Its object was only to accompany the entrance, so there was no reason for going on with it after the celebrant had arrived at the altar. Already in the First Roman Ordo as soon as the pope is ready to begin Mass he signs to the choir-master to leave out the rest of the psalm and go on at once to the Gloria Patri (ed. Atchley, p. 128). Since the early Middle Ages the psalm has been further shortened to one verse (Durandus, "Rationale", IV, 5). So it received the form it still has, namely: an antiphon, one verse of a psalm, Gloria Patri, Sicut erat, the antiphon repeated. In the Milanese Rite the antiphon of the Ingressa is not repeated except in Requiem Masses; on the other hand, in some medieval uses it was repeated several times (Durandus, loc. cit.). On great feasts the Carmelites still repeat it twice at the end. The antiphon is taken as a rule from the Psalter (Durandus calls such introits regulares); sometimes (e.g. second and third Christmas Mass, Ascension-Day, Whit-Sunday, etc.) from another part of the Bible; more rarely (Assumption, All Saints, many Masses of Our Lady — "Salve sancta parens", Requiems, etc.) it is a composition by some later writer. The verse of the psalm in the earlier introits is the first (obviously still a fragment of the whole), except that when the antiphon itself is the first verse the "psalm" is the next (twelfth and fifteenth Sundays after Pentecost, etc.). In later times it has become common to choose a suitable verse regardless of this rule (e. g. the Crown of Thorns Mass for Friday after Ash Wednesday, St. Ignatius Loyola on 31 July, etc.). The text of the psalms used in the introits (as throughout the Missal) is not the Vulgate but the Itala. In Paschal time two Alleluias are added to the antiphon, sometimes (Easter Day, Low Sunday, the Third and Fourth Sundays after Easter, etc.) there are three. In Requiems and Masses de tempore in Passiontide, when the Psalm Judica is not said, there is no Gloria Patri at the Introit. On Holy Saturday and at the chief Mass on Whitsun Eve (when the prophecies are read) there is no Introit at all. The reason of this is obvious. The Introit accompanies the entrance; but on these occasions the celebrant has been at the altar for some time before Mass begins. We name Masses (that is the complex of changeable prayers that make up the Puerperium) from the first words of the Introit by which they begin. Thus the Mass for the first Sunday of Advent is called Ad te leva; the two Masses of the Sacred Heart are distinguished as Miserability and Exordium; a Mass for the dead is spoken of as a Requiem, and so on. There is nothing corresponding to our Introit in the Eastern rites. In all of them the liturgy begins quite differently. The preparation (vesting, preparation of the offerings) takes place in the sanctuary, so there is no procession to the altar.
RITUAL OF THE INTROIT
At high (or sung) Mass till quite lately the rule had obtained that the choir did not begin the Introit till the celebrant began the first prayers at the foot of the altar. Now the new Vatican "Gradual" (1908) has restored the old principle, that it is to be sung while the procession moves from the sacristy to the altar. ("De redivivus servanda in cant miss" in the introduction.) It should therefore be begun as soon as the head of the procession appears in the church. One or more cantors sing to the sign*, all continue; the cantors alone sing the first half of the psalm and the V. Gloria Patri (ibid.). The celebrant, having finished the preparatory prayers at the altar-steps, goes up to the altar and kisses it (saying meanwhile the two short prayers, A ufer a nobis and Oranges te); then, going to the left (Epistle) side, he reads from the Missal the Introit, just as it is sung. This is one of the continual reactions of low Mass on high Mass. When the custom of low Mass began (in the early Middle Ages) the celebrant had to supply all the parts of deacon, subdeacon, and choir himself. Then, as he became used to saying these parts, he said them even at high Mass, too; they were, besides, chanted by others. So the rule has obtained that everything is said by the celebrant. The recital of the Introit should be considered as the real beginning of Mass, since what has gone before is rather of the nature of the celebrant's preparation. For this reason he makes the sign of the cross at its first words, according to the general rule of beginning all solemn functions (in this case the Mass) with that sign. At Requiem Masses he makes the cross not on himself but over the Missal, quasi aliquem benedicens says the rubric (Ritus cel., xiii, 1). This is understood as directing the blessing to the souls in purgatory. At low Mass there is no change here, save the omission of the chant by the choir.
Of the medieval commentators see especially DURANDUS, Rationale Divinorum Officiorum, IV, 5; BENEDICT XIV, De S. Miss Sacrificio, II 4; DuCHESNE, Origines du culte chrétien (Paris, 1898), 154-155; GIHR, Das heilige Messopfer (Freiburg im Br., 1897), 346-57.