1. When Christ the Lord was about to celebrate the passover meal with His disciples and institute the sacrifice of His body and blood, He directed them to prepare a large room, arranged for the supper (Lk 22:12). The Church has always regarded this command of Christ as applying to itself when it gives directions about the preparation of the sentiments of the worshipers, the place, rites, and texts for the celebration of the eucharist. The current norms, laid down on the basis of the intent of Vatican Council II, and the new Missal that will be used henceforth in the celebration of Mass by the Church of the Roman Rite, are fresh evidence of the great care, faith, and unchanged love that the Church shows toward the Eucharist. They attest as well to its coherent tradition, continuing amid the introduction of some new elements.
A Witness to Unchanged Faith
2. The sacrificial nature of the Mass was solemnly proclaimed by the Council of Trent in agreement with the whole tradition of the Church.  Vatican Council II reaffirmed this teaching in these significant words: "At the Last Supper our Savior instituted the eucharistic sacrifice of His body and blood. He did this in order to perpetuate the sacrifice of the cross throughout the centuries until he should come again and in this way to entrust to his beloved Bride, the Church, a memorial of his death and resurrection." 
The Council's teaching is expressed constantly in the formularies of the Mass. This teaching, in the concise words of the Leonine Sacramentary, is that "the work of our redemption is carried out whenever we celebrate the memory of this sacrifice";  it is aptly and accurately brought out in the eucharistic prayers. At the anamnesis or memorial, the priest, addressing God in the name of all the people, offers in thanksgiving the holy and living sacrifice: the Church's offering and the Victim whose death has reconciled us with God.  The priest also prays that the body and blood of Christ may be a sacrifice acceptable to the Father, bringing salvation to the whole world. 
In this new Missal, then, the Church's rule of prayer (lex orandi) corresponds to its constant rule of faith (lex credendi). This rule of faith instructs us that the sacrifice of the cross and its sacramental renewal in the Mass, which Christ instituted at the Last Supper and commanded His apostles to do in His memory, are one and the same, differing only in the manner of offering and that consequently the Mass is at once a sacrifice of praise and thanksgiving, of reconciliation and expiation.
3. The celebration of Mass also proclaims the sublime mystery of the Lord's real presence under the eucharistic elements, which Vatican Council II  and other documents of the Church's magisterium  have reaffirmed in the same sense and as the same teaching that the Council of Trent had proposed as a matter of faith.  The Mass does this not only by means of the very words of consecration, by which Christ becomes present through transubstantiation, but also by that spirit and expression of reverence and adoration in which the eucharistic liturgy is carried out. For the same reason the Christian people are invited in Holy Week on Holy Thursday and on the solemnity of Corpus Christi to honor this wonderful sacrament in a special way by their adoration.
4. Further, because of the priest's more prominent place and office in the rite, its form sheds light on the ministerial priesthood proper to the presbyter, who offers the sacrifice in the person of Christ and presides over the assembly of a holy people. The meaning of his office is declared and detailed in the preface for the chrism Mass on Thursday of Holy Week, the day celebrating the institution of the priesthood. The preface brings out the passing on of the sacredotal power. It is the continuation of the power of Christ, High Priest of the New Testament.
5. In addition, the ministerial priesthood puts into its proper light another reality of which much should be made, namely, the royal priesthood of believers. Through the ministry of presbyters the people's spiritual sacrifice to God is brought to completeness in union with the sacrifice of Christ, our one and only Mediator.  For the celebration of the Eucharist is the action of the whole Church; in it all should do only, but all of, those parts that belong to them in virtue of their place within the people of God. In this way greater attention will be given to some aspects of the Eucharistic celebration that have sometimes been neglected in the course of time. For these people are the people of God, purchased by Christ's blood, gathered together by the Lord, nourished by His word. They are a people called to offer God the prayers of the entire human family, a people giving thanks in Christ for the mystery of salvation by offering His sacrifice. Finally, they are a people growing together into unity by sharing in Christ's body and blood. These people are holy by their origin, but becoming ever more holy by conscious, active, and fruitful participation in the mystery of the Eucharist. 
A Witness to Unbroken Tradition
6. In setting forth its decrees for the revision of the Order of Mass, Vatican Council II directed, among other things, that some rites be restored "to the vigor they had in the tradition of the Fathers";  this is a quotation from the Apostolic Constitution Quo primum of 1570, by which St. Pius V promulgated the Tridentine Missal. The fact that the same words are used in reference to both Roman Missals indicates how both of them, although separated by four centuries, embrace one and the same tradition. And when the more profound elements of this tradition are considered, it becomes clear how remarkably and harmoniously this new Roman Missal improves on the older one.
7. The older Missal belonged to the difficult period of attacks against Catholic teaching on the sacrificial nature of the Mass, the ministerial priesthood, and the real and permanent presence of Christ under the eucharistic elements. St. Pius V was therefore especially concerned with preserving the relatively recent developments in the Church's tradition, then unjustly being assailed, and introduced only very slight changes into the sacred rites. In fact, the Roman Missal of 1570 differs very little from the first printed edition of 1474, which in turn faithfully follows the Missal used at the time of Pope Innocent III (1198-1216). Manuscripts in the Vatican Library provided some verbal emendations, but they seldom allowed research into "ancient and approved authors" to extend beyond the examination of a few liturgical commentaries of the Middle Ages.
8. Today, on the other hand, countless studies of scholars have enriched the "tradition of the Fathers" that the revisers of the Missal under St. Pius V followed. After the Gregorian Sacramentary was first published in 1571, many critical editions of other ancient Roman and Ambrosian sacramentaries appeared. Ancient Spanish and Gallican liturgical books also became available, bringing to light many prayers of profound spirituality that had hitherto been unknown.
Traditions dating back to the first centuries before the formation of the Eastern and Western rites are also better known today because so many liturgical documents have been discovered.
The continuing progress in patristic studies has also illuminated eucharistic theology through the teachings of such illustrious saints of Christian antiquity as Irenaeus, Ambrose, Cyril of Jerusalem, and John Chrysostom.
9. The "tradition of the Fathers" does not require merely the preservation of what our immediate predecessors have passed on to us. There must also be profound study and understanding of the Church's entire past and of all the ways in which its single faith has been expressed in the quite diverse human and social forms prevailing in Semitic, Greek, and Latin cultures. This broader view shows us how the Holy Spirit endows the people of God with a marvelous fidelity in preserving the deposit of faith unchanged, even though prayers and rites differ so greatly.
Adaptation to Modern Conditions
10. As it bears witness to the Roman Church's rule of prayer (lex orandi) and guards the deposit of faith handed down by the later councils, the new Roman Missal in turn marks a major step forward in liturgical tradition.
The Fathers of Vatican Council II in reaffirming the dogmatic statements of the Council of Trent were speaking at a far different time in the world's history. They were able therefore to bring forward proposals and measures of a pastoral nature that could not have even been foreseen four centuries ago.
11. The Council of Trent recognized the great catechetical value of the celebration of Mass, but were unable to bring out all its consequences for the actual life of the Church. Many were pressing for permission to use the vernacular in celebrating the eucharistic sacrifice, but the Council, judging the conditions of that age, felt bound to answer such a request with a reaffirmation of the Church's traditional teaching. This teaching is that the eucharistic sacrifice is, first and foremost, the action of Christ Himself and therefore the manner in which the faithful take part in the Mass does not affect the efficacy belonging to it. The Council thus stated in firm but measured words: "Although the Mass contains much instruction for the faithful, it did not seem expedient to the Fathers that as a general rule it be celebrated in the vernacular."  The Council accordingly anathemized anyone maintaining that "the rite of the Roman Church, in which part of the canon and the words of consecration are spoken in a low voice, should be condemned or that the Mass must be celebrated only in the vernacular."  Although the Council of Trent on the one hand prohibited the use of the vernacular in the Mass, nevertheless, on the other, it did direct pastors to substitute appropriate catechesis: "Lest Christ's flock go hungry . . . the Council commands pastors and others having the care of souls that either personally or through others they frequently give instructions during Mass, especially on Sundays and holy days, on what is read at Mass and that among their instructions they include some explanation of the mystery of this sacrifice." 
12. Convened in order to adapt the Church to the contemporary requirements of its apostolic task, Vatican Council II examined thoroughly, as had Trent, the pedagogic and pastoral character of the liturgy.  Since no Catholic would now deny the lawfulness and efficacy of a sacred rite celebrated in Latin, the Council was able to acknowledge that "the use of the mother tongue frequently may be of great advantage to the people" and gave permission for its use.  The enthusiasm in response to this decision was so great that, under the leadership of the bishops and the Apostolic See, it has resulted in the permission for all liturgical celebrations in which the faithful participate to be in the vernacular for the sake of a better comprehension of the mystery being celebrated.
13. The use of the vernacular in the liturgy may certainly be considered an important means for presenting more clearly the catechesis on the mystery that is part of the celebration itself. Nevertheless, Vatican Council II also ordered the observance of certain directives, prescribed by the Council of Trent but not obeyed everywhere. Among these are the obligatory homily on Sundays and holydays  and the permission to interpose some commentary during the sacred rites themselves. 
Above all, Vatican Council II strongly endorsed "that more complete form of participation in the Mass by which the faithful, after the priest's communion, receive the Lord's body from the same sacrifice."  Thus the Council gave impetus to the fulfillment of the further desire of the Fathers of Trent that for fuller participation in the Holy Eucharist "the faithful present at each Mass should communicate not only by spiritual desire but also by sacramental communion." 
14. Moved by the same spirit and pastoral concern, Vatican Council II was able to reevaluate the Tridentine norm on Communion under both kinds. No one today challenges the doctrinal principles on the completeness of eucharistic Communion under the form of bread alone. The Council thus gave permission for the reception of Communion under both kinds on some occasions, because this more explicit form of the sacramental sign offers a special means of deepening the understanding of the mystery in which the faithful are taking part. 
15. Thus the Church remains faithful in its responsibility as teacher of truth to guard "things old," that is, the deposit of tradition; at the same time it fulfills another duty, that of examining and prudently bringing forth "things new" (see Mt 13:52).
Accordingly, a part of the new Roman Missal directs the prayer of the Church expressly to the needs of our times. This is above all true of the ritual Masses and the Masses for various needs and occasions, which happily combine the traditional and the contemporary. Thus many expressions, drawn from the Church's most ancient tradition and become familiar through the many editions of the Roman Missal, have remained unchanged. Other expressions, however, have been adapted to today's needs and circumstances and still others - for example, the prayers for the Church, the laity, the sanctification of human works, the community of all people, certain needs proper to our era - are completely new compositions, drawing on the thoughts and even the very language of the recent conciliar documents.
The same awareness of the present state of the world also influenced the use of texts from very ancient traditions. It seemed that this cherished treasure would not be harmed if some phrases were changed so that the style of language would be more in accord with the language of modern theology and would faithfully reflect the actual state of the Church's discipline. Thus there have been changes of some expressions bearing on the evaluation and use of the good things of the earth and of allusions to a particular form of outward penance belonging to another age in the history of the Church.
In short, the liturgical norms of the Council of Trent have been completed and improved in many respects by those of Vatican Council II. This Council has brought to realization the efforts of the last four hundred years to move the faithful closer to the sacred liturgy, especially the efforts of recent times and above all the zeal for the liturgy promoted by St. Pius X and his successors.