Novus Ordo Missae

The "Novus Ordo Missae" (New Order of the Mass) is the form of the Roman Catholic Mass introduced by Pope Paul VI in 1969 after the Second Vatican Council to replace the Tridentine rite of Mass (from the Tridentine Missal promulgated by the Quo Primum bull (1570) of Pope Pius V following the Council of Trent). The revised Roman Missal approved by Pope Paul VI and introducing the reforms was written by a team of experts after the end of the council and contains certain changes to the rites and allows, but does not encourage, vernacular translations to be used.

A number of Catholics, such as those of Una Voce[?] or followers of Sedevacantism, believe that a Mass according to the new Roman Missal is an "invalid Mass", as it supposedly goes against the bulls of previous popes and so contains changes that they would regard as heretical. Other opponents link the decline of the Church to the "collapse" of the liturgy.