I answer that, Since the whole mystery of our salvation is comprised in this sacrament, therefore is it performed with greater solemnity than the other sacraments. And since it is written (Eccles. 4:17): "Keep thy foot when thou goest into the house of God"; and (Ecclus. 18:23): "Before prayer prepare thy soul," therefore the celebration of this mystery is preceded by a certain preparation in order that we may perform worthily that which follows after. The first part of this preparation is Divine praise, and consists in the "Introit": according to Ps. 49:23: "The sacrifice of praise shall glorify me; and there is the way by which I will show him the salvation of God": and this is taken for the most part from the Psalms, or, at least, is sung with a Psalm, because, as Dionysius says (Eccl. Hier. iii): "The Psalms comprise by way of praise whatever is contained in Sacred Scripture."
The second part contains a reference to our present misery, by reason of which we pray for mercy, saying: "Lord, have mercy on us," thrice for the Person of the Father, and "Christ, have mercy on us," thrice for the Person of the Son, and "Lord, have mercy on us," thrice for the Person of the Holy Ghost; against the threefold misery of ignorance, sin, and punishment; or else to express the "circuminsession" of all the Divine Persons.
The third part commemorates the heavenly glory, to the possession of which, after this life of misery, we are tending, in the words, "Glory be to God on high," which are sung on festival days, on which the heavenly glory is commemorated, but are omitted in those sorrowful offices which commemorate our unhappy state.
The fourth part contains the prayer which the priest makes for the people, that they may be made worthy of such great mysteries.
There precedes, in the second place, the instruction of the faithful, because this sacrament is "a mystery of faith," as stated above (Question [78], Article [3], ad 5). Now this instruction is given "dispositively," when the Lectors and Sub-deacons read aloud in the church the teachings of the prophets and apostles: after this "lesson," the choir sing the "Gradual," which signifies progress in life; then the "Alleluia" is intoned, and this denotes spiritual joy; or in mournful offices the "Tract", expressive of spiritual sighing; for all these things ought to result from the aforesaid teaching. But the people are instructed "perfectly" by Christ's teaching contained in the Gospel, which is read by the higher ministers, that is, by the Deacons. And because we believe Christ as the Divine truth, according to Jn. 8:46, "If I tell you the truth, why do you not believe Me?" after the Gospel has been read, the "Creed" is sung in which the people show that they assent by faith to Christ's doctrine. And it is sung on those festivals of which mention is made therein, as on the festivals of Christ, of the Blessed Virgin, and of the apostles, who laid the foundations of this faith, and on other such days.
So then, after the people have been prepared and instructed, the next step is to proceed to the celebration of the mystery, which is both offered as a sacrifice, and consecrated and received as a sacrament: since first we have the oblation; then the consecration of the matter offered; and thirdly, its reception.
In regard to the oblation, two things are done, namely, the people's praise in singing the "offertory," expressing the joy of the offerers, and the priest's prayer asking for the people's oblation to be made acceptable to God. Hence David said (1 Para 29:17): "In the simplicity of my heart, I have . . . offered all these things: and I have seen with great joy Thy people which are here present, offer Thee their offerings": and then he makes the following prayer: "O Lord God . . . keep . . . this will."
Then, regarding the consecration, performed by supernatural power, the people are first of all excited to devotion in the "Preface," hence they are admonished "to lift up their hearts to the Lord," and therefore when the "Preface" is ended the people devoutly praise Christ's Godhead, saying with the angels: "Holy, Holy, Holy"; and His humanity, saying with the children: "Blessed is he that cometh." In the next place the priest makes a "commemoration," first of those for whom this sacrifice is offered, namely, for the whole Church, and "for those set in high places" (1 Tim. 2:2), and, in a special manner, of them "who offer, or for whom the mass is offered." Secondly, he commemorates the saints, invoking their patronage for those mentioned above, when he says: "Communicating with, and honoring the memory," etc. Thirdly, he concludes the petition when he says: "Wherefore that this oblation," etc., in order that the oblation may be salutary to them for whom it is offered.
Then he comes to the consecration itself. Here he asks first of all for the effect of the consecration, when he says: "Which oblation do Thou, O God," etc. Secondly, he performs the consecration using our Saviour's words, when he says: "Who the day before," etc. Thirdly, he makes excuse for his presumption in obeying Christ's command, saying: "Wherefore, calling to mind," etc. Fourthly, he asks that the sacrifice accomplished may find favor with God, when he says: "Look down upon them with a propitious," etc. Fifthly, he begs for the effect of this sacrifice and sacrament, first for the partakers, saying: "We humbly beseech Thee"; then for the dead, who can no longer receive it, saying: "Be mindful also, O Lord," etc.; thirdly, for the priests themselves who offer, saying: "And to us sinners," etc.
Then follows the act of receiving the sacrament. First of all, the people are prepared for Communion; first, by the common prayer of the congregation, which is the Lord's Prayer, in which we ask for our daily bread to be given us; and also by private prayer, which the priest puts up specially for the people, when he says: "Deliver us, we beseech Thee, O Lord," etc. Secondly, the people are prepared by the "Pax" which is given with the words, "Lamb of God," etc., because this is the sacrament of unity and peace, as stated above (Question [73], Article [4]; Question [79], Article [1]). But in masses for the dead, in which the sacrifice is offered not for present peace, but for the repose of the dead, the "Pax" is omitted.
Then follows the reception of the sacrament, the priest receiving first, and afterwards giving it to others, because, as Dionysius says (Eccl. Hier. iii), he who gives Divine things to others, ought first to partake thereof himself.
Finally, the whole celebration of mass ends with the thanksgiving, the people rejoicing for having received the mystery (and this is the meaning of the singing after the Communion); and the priest returning thanks by prayer, as Christ, at the close of the supper with His disciples, "said a hymn" (Mt. 26:30).