Part III: Guidelines for the Renewal of Liturgical Life
10. From these principles are derived certain norms and guidelines which must govern the renewal of liturgical life. While the reform of the liturgy desired by the Second Vatican Council can be considered already in progress, the pastoral promotion of the liturgy constitutes a permanent commitment to draw ever more abundantly from the riches of the liturgy that vital force which spreads from Christ to the members of His body, which is the Church.
Since the liturgy is the exercise of the priesthood of Christ, it is necessary to keep ever alive the affirmation of the disciple faced with the mysterious presence of Christ: "It is the Lord!" (Jn. 21:7). Nothing of what we do in the liturgy can appear more important than what in an unseen but real manner Christ accomplishes by the power of His Spirit. A faith alive in charity, adoration, praise of the Father and silent contemplation will always be the prime objective of liturgical and sacramental pastoral care.
Since the liturgy is totally permeated by the word of God, any other word must be in harmony with it, above all in the homily but also in various interventions of the ministers and in the hymns which are sung. No other reading may supplant the biblical word, and the words of men must be at the service of the word of God without obscuring it.
Since liturgical celebrations are not private acts but "celebrations of the Church, the 'sacrament of unity,'"  their regulation is dependent solely upon the hierarchical authority of the Church.  The liturgy belongs to the whole body of the Church.  It is for this reason that it is not permitted to anyone, even a priest, or any group to add, subtract or change anything whatsoever on their own initiative.  Fidelity to the rites and to the authentic texts of the liturgy is a requirement of the lex orandi, which must always be conformity with the lex credendi. A lack of fidelity on this point may even affect the very validity of the sacraments.
Since it is a celebration of the Church, the liturgy requires the active, conscious and full participation of all, according to the diversity of orders and office.  All, the ministers and the other faithful, in the accomplishment of their particular function, do that and only that which is proper to them.  It is for this reason that the Church gives preference to celebrations in common when the nature of the rites implies this;  she encourages the formation of ministers, readers, cantors and commentators, who carry out a true liturgical ministry;  she has restored concelebration,  and she recommends the common celebration of the Liturgy of the Hours. 
Given that the liturgy is schooled of the prayer of the Church, it has been considered good to introduce and develop the use of the vernacular - without diminishing the use of Latin, retained by the council for the Latin rite  - so that every individual can understand and proclaim in his or her mother tongue the wonders of God (cf. Acts 2:11). It has likewise been considered good to increase the number of prefaces and eucharistic prayers, so as to enrich the Church's treasury of prayer and an understanding of the mystery of Christ.
Since the liturgy has great pastoral value, the liturgical books have provided for a certain degree of adaptation to the assembly and to individuals, with the possibility of openness to the traditions and culture of different peoples.  The revision of the rites has sought a noble simplicity  and signs that are easily understood, but the desired simplicity must not degenerate into an impoverishment of the signs. On the contrary, the signs, above all the sacramental signs, must be easily grasped but carry the greatest possible expressiveness. Bread and wine, water and oil, and also incense, ashes, fire and flowers, and indeed almost all the elements of creation have their place in the liturgy as gifts to the Creator and as a contribution to the dignity and beauty of the celebration.