Marcel Lefebvre (born Tourcoing, France, November 29, 1905; died Martigny, Switzerland, March 25, 1991), was a leader of those opposed to the reforms of the Second Vatican Council, especially the abolition of the Tridentine Mass, and to the saying of Mass in the vernacular.

By the early 1960s he had risen to high office within the Roman Catholic Church, including Archbishop, Assistant to the Papal Throne and Roman Count, and General Superior of the Holy Ghost Fathers, when in 1962 he was appointed by the Pope to the Preparatory Commission for the Second Vatican Council. There, he became opposed to the reformist plans of the other members of the Commission, many of which would later be adopted by the Council.

After the Second Vatican Council, he became increasingly opposed to the direction that Catholic church was taking; and for this opposition he was being progressively sidelined and removed from his positions of authority within the church. In 1970 he opened a small seminary in Fribourg, Switzerland, convinced that the other existing Catholic seminaries were falling into heresy; and established there the "Priestly Fraternity of Saint Pius X", which became the Society of St. Pius X (SSPX).

After the promulgation of the Novus Ordo Mass by the Pope in 1969, Lefebvre established a group of theologians to criticize it; however, although their criticisms received the support of several Cardinals, the Vatican did not respond to it, and the majority of the Church hierarchy became increasingly opposed to Lefebvre's actions. In 1971, Lefebvre announced to his seminarians his rejection of the Novus Ordo liturgy.

The Vatican and Lefebvre engaged in dialogues but reached no consensus. In 1975 an attempt was made to prohibit the SSPX, and then in 1976 the Pope publicly condemned Lefebvre for his refusal to the accept the Novus Ordo Mass. Finally, later that same year, Lefebvre was forbidden from saying Mass.

Meanwhile, the SSPX opened seminaries in a number of countries around the world, and Lefebvre gained for himself a large following of traditionalist Catholics. Lefebvre and the Vatican engaged in dialogue, with Lefebvre meeting first with Paul VI and then with John Paul II. This dialogue did not, however, overcome their disagreements.

Finally, in 1988, Lefebvre consecrated four bishops, without permission from Rome. In response, the Vatican declared him excommunicated by virtue of performing a schismatic act.

See also Traditional Catholic