Mediator Dei, a papal encyclical, was issued by Pope Pius XII in 1947. The encyclical suggests new directions and active participation instead of a merely passive role of the faithful in the liturgy, in liturgical ceremonies and in the life of their parish. The encyclical also emphasizes the importance of the Eucharistic cult. Mediator Dei is one of the more important encyclicals of Pope Pius XII. The encyclical condemned certain excesses of liturgical reform and stressed the importance of the union of sacrifice and altar with communion, which greatly directed additional reforms during Vatican II.
Pope Pius defends liturgy as important, sacred and sacramental. Liturgy is more than the sum of liturgical actions and prescriptions. It is an error, consequently, and a mistake to think of the sacred liturgy as merely the outward or visible part of divine worship or as an ornamental ceremonial. No less erroneous is the notion that it consists solely in a list of laws and prescriptions according to which the ecclesiastical hierarchy orders the sacred rites to be performed. The encyclical has four parts.
Nature of liturgy
The first part explains the nature, origin and development of liturgy. Liturgy is public worship, an obligation for individuals and communities. Liturgy is outward adoration of God as well as a fountain for personal piety. It originated with the early Church:
The first Christians ... "were persevering the doctrine of the apostles and in communication by the breaking of bread and their prayers."  Whenever their pastors could summon a little group of the faithful together, they set up an altar on which they proceeded to offer the sacrifice, and around which were ranged all other rites appropriate for the saving of souls and for the honor due to God. 
Through prayer, the members of the Mystical Body of Christ are harmonized and united. The liturgy is regulated by the clergy and hierarchy of the Church. It has divine and human elements. Its human elements result from the teachings of the Church, Church laws, pious usages by the faithful and the development of art and music. It must also be noted, however, that Pius warns strongly against a sense of "liturgical archaeologism", the belief that the most ancient practices are more venerable. He recognizes that liturgy is organic, and to return to the most ancient practices would ignore centuries of liturgical organic development. The Holy Father goes on to say that
"[One] would be straying from the straight path were he to wish the altar restored to its primitive table form; were he to want black excluded as a color for liturgical vestments; were he to forbid the use of sacred images and statues in Church; were he to order the crucifix so designed that the Divine Redeemer's Body shows no trace of his cruel sufferings." 
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The Eucharist is a renewal of the sacrifice on the cross. Christ is the Priest, the Sacrifice and the Purpose of the Eucharistic sacrifice. The faithful should participate but they do not have priestly authority. They participate in the sacrifice together with the priest. They participate by cleansing the souls of arrogance, anger, guilt, lust and other sins, and thus see more clearly the picture of Christ in themselves.
Mediator Dei advises the bishops to create offices to encourage active participation and dignified services, and to ensure, that individual priests do not use the Eucharist as experiments for their own purposes. The encyclical encourages the faithful to participate in Holy Communion and uses the terms, spiritual and sacramental communion. Communion must be followed by a thanksgiving.
The encyclical encourages the adoration of the Blessed Sacrament and in Eucharistic Blessings. The historical Jesus and the Eucharist cannot be separated. The full liturgy opens to the faithful the mystery of the cross, are in the likeness of their Redeemer.
All the elements of the liturgy, then, would have us reproduce in our hearts the likeness of the divine Redeemer through the mystery of the cross, according to the words of the Apostle of the Gentiles, "With Christ I am nailed to the cross. I live, now not I, but Christ live in me." Gal. 2:19-20. Thus we become a victim, as it were, along with Christ to increase the glory of the eternal Father.
Pius XII opposed excesses in "Mediator Dei":
Assuredly it is a wise and most laudable thing to return in spirit and affection to the sources of the sacred liturgy. For research in this field of study, by tracing it back to its origins, contributes valuable assistance towards a more thorough and careful investigation of the significance of feast-days, and of the meaning of the texts and sacred ceremonies employed on their occasion.
But it is neither wise nor laudable to reduce everything to antiquity by every possible device. Thus, to cite some instances, one would be straying from the straight path were he to wish the altar restored to its primitive tableform; were he to want black excluded as a color for the liturgical vestments; were he to forbid the use of sacred images and statues in Churches; were he to order the crucifix so designed that the divine Redeemer's body shows no trace of His cruel sufferings; and lastly were he to disdain and reject polyphonic music or singing in parts, even where it conforms to regulations issued by the Holy See.
Pius XII stated that exaggerated reforms have harmful effects on Catholic spirituality: This way of acting bids fair to revive the exaggerated and senseless antiquarianism to which the illegal Council of Pistoia gave rise. It likewise attempts to reinstate a series of errors which were responsible for the calling of that meeting as well as for those resulting from it, with grievous harm to souls, and which the Church, the ever watchful guardian of the "deposit of faith" committed to her charge by her divine Founder, had every right and reason to condemn. For perverse designs and ventures of this sort tend to paralyze and weaken that process of sanctification by which the sacred liturgy directs the sons of adoption to their Heavenly Father of their souls' salvation.
Liturgy of the Hours
The Liturgy of the Hours (Divine Office) is a never-ending prayer of the Church, requiring a spirit of contemplation. The faithful are encouraged to participate in the Liturgy of the Hours especially during Sunday. Thus they participate in the life of Christ, which the Church repeats and explains on a yearly basis.
The saints are ideals and models and intercessors. Among the inhabitants of heaven, Mary has a special place. She is like no other involved in the mysteries of Christ. She offers her son and provides all help needed.
Liturgy and the arts
Mediator Dei discusses the need to have tasteful and beautiful houses of worship. Not necessarily rich in historical artifacts, but clean and not overburdened with Kitsch. While Pius XII was critical of those who stripped their churches of virtually all ancient art (including pictures and statuary), he disapproved of churches and altars overloaded with art-like trinkets and works.
Nevertheless, in keeping with the duty of Our office, We cannot help deploring and condemning those works of art, recently introduced by some, which seem to be a distortion and perversion of true art and which at times openly shock Christian taste, modesty and devotion, and shamefully offend the true religious sense. These must be entirely excluded and banished from our churches, like "anything else that is not in keeping with the sanctity of the place."
Progressive theological elements of Modernism, many of whom were subsequently censured and silenced after Pius XII's promulgation of Humani generis in 1950, especially in France, (Henri de Lubac, Yves Congar) presented a highly colored interpretation of the encyclical. Nouvelle Revue Théologique labelled Mediator Dei "the most important teaching, which the Magisterium ever issued, and one of greatest documents of this pontificate".
The Jesuit G.M. Hansens writes in the influential Civiltà Cattolica, ... that the importance of the encyclical are obviously the reforms, but also the notion, that without liturgy, religious life is not possible, and, liturgy as Pius XII taught, is “more than a beautiful spectacle but ultimate adoration of God Himself”. An unsigned editorial in the journal Life of the Spirit opines that some eccentric German liturgical reformists went too far liturgically, “and are now told to look at the depths of the dogma”. A Blackfriers commentary also points to liturgical developments in Germany, which are now largely legal.
An American journal, Orate Fratres, issued by Saint John's Abbey in Minnesota, writes that with this encyclical of Pope Pius XII, liturgy ceases to be an unimportant composite of ceremonies and regulations. It is now accepted dogma, that liturgy is not static but actively changing lives. It is hoped that the encyclical will help to coordinate the liturgical renewal throughout the Catholic world and thus multiply its effects on Christian life everywhere, similarly to the reform movement of Cluny or the reforms of the Council of Trent.
Highlights of the encyclical
- Many of the faithful are unable to use the Roman missal even though it is written in the vernacular; nor are all capable of understanding correctly the liturgical rites and formulas. (108)
- So varied and diverse are men's talents and characters that it is impossible for all to be moved and attracted to the same extent by community prayers, hymns and liturgical services.
- Moreover, the needs and inclinations of all are not the same, nor are they always constant in the same individual.
- Who, then, would say, on account of such a prejudice, that all these Christians cannot participate in the Mass nor share its fruits?
- On the contrary, they can adopt some other method which proves easier for certain people; for instance, they can lovingly meditate on the mysteries of Jesus Christ or perform other exercises of piety or recite prayers which, though they differ from the sacred rites, are still essentially in harmony with them.
- For we must always live in Christ and give ourselves to Him completely, so that in Him, with Him and through Him the heavenly Father may be duly glorified. The sacred liturgy requires, however, that both of these elements be intimately linked with each another. This recommendation the liturgy itself is careful to repeat, as often as it prescribes an exterior act of worship. Thus we are urged, when there is question of fasting, for example, "to give interior effect to our outward observance." Otherwise religion clearly amounts to mere formalism, without meaning and without content.
- You recall .. how the divine Master expels from the sacred temple, as unworthy to worship there, people who pretend to honor God with nothing but neat and good sounding phrases, like actors in a theater, and think themselves perfectly capable of working out their eternal salvation without plucking their vices from their hearts. It is, therefore, the keen desire of the Church that all of the faithful kneel at the feet of the Redeemer to tell Him how much they venerate and love Him 
- The sacraments and the sacrifice of the altar, being Christ's own actions, must be capable in themselves of conveying and dispensing grace from the divine Head to the members of the Mystical Body. But if they are to produce their proper effect, it is absolutely necessary that our hearts be properly disposed to receive them. Hence the warning of Paul the Apostle with reference to holy communion, "But let a man first prove himself; and then let him eat of this bread and drink of the chalice."
- They, therefore, err from the path of truth who do not want to have Masses celebrated unless the faithful communicate; and those are still more in error who, in holding that it is altogether necessary for the faithful to receive holy communion as well as the priest, put forward the captious argument that here there is question not of a sacrifice merely, but of a sacrifice and a supper of brotherly union, and consider the general communion of all present as the culminating point of the whole celebration.
- In Holy Week, when the most bitter sufferings of Jesus Christ are put before us by the liturgy, the Church invites us to come to Calvary and follow in the blood-stained footsteps of the divine Redeemer, to carry the cross willingly with Him, to reproduce in our own hearts His spirit of expiation and atonement, and to die together with Him.
- Pope Pius XII enc, Mediator Dei, 25
- Acts, 2:42.
- Pope Pius XII enc, Mediator Dei, 21
- Pope Pius XII enc, Mediator Dei, 32
- Pope Pius XII enc, Mediator Dei, 62
- Pope Pius XII enc, Mediator Dei, 63
- Pope Pius XII enc, Mediator Dei, 84.
- Pope Pius XII enc, Mediator Dei, 102
- Mediator Dei 62
- Mediator Dei 64
- Pope Pius XII enc, Mediator Dei, 124
- Pope Pius XII enc, Mediator Dei, 137.
- Mediator Dei, 195.
- Nouvelle Revue Théologique, 1948.
- G.M.Hanssens, "Mediator Dei", Civilta Catolica, March 1948 and May 1948.
- "Mediator Dei", Life of the Spirit, April 1948.
- "Mediator Dei", Blackfriers, May 1948.
- W. Busch, "Mediator Dei", Orate Fratres, St. John’s Abbey, Collegeville, Minnesota
- Roman Missal, Secret for Thursday after the Second Sunday of Lent
- Pope Pius XII enc, Mediator Dei, 24
- Cf. Mark, 7:6 and Isaiah, 29:13.
- 1 Cor.11:28.
- Roman Missal, Ash Wednesday; Prayer after the imposition of ashes.
- Pope Pius XII enc, Mediator Dei, 31
- Mediator Dei, 114
- Pope Pius XII enc, Mediator Dei, 158