Michel-Louis Guérard des Lauriers

Michel-Louis Guérard des Lauriers (25 October 1898 – 27 February 1988) was a French Catholic clergyman and theologian who was a member of the Dominican Order but was eventually excommunicated from the Catholic Church. He was involved in contemporary traditionalist Catholicism, both in France and the wider world, especially in support of the Tridentine Mass. An early adopter of sedevacantism, a theory claiming that Pope Paul VI was not a legitimate Pope, he changed his position by 1979, creating a new theory known as sedeprivationism. Also called the ‘Cassiacum Thesis’, it is sometimes considered adjacent to or a sub-set of sedevacantism.

Ordained to the priesthood in 1931, he was a Thomist theologian in France and was noted for his involvement in preparatory work for the dogmatic definition of the Assumption of Mary, officially defined and proclaimed as infallible doctrine by Pope Pius XII in 1950. He was involved in the theological controversies of the time and was an opponent of the emerging Nouvelle théologie, particularly against Pierre Teilhard de Chardin and Henri de Lubac. During the early 1960s, he was a lecturer at the Pontifical Lateran University in Rome and a member of the Pontifical Academy of Saint Thomas Aquinas.

Concerned with the changes associated with the Second Vatican Council and the introduction of the New Order of Mass, he was one of the writers of the Short Critical Study of the Novus Ordo Missae in 1969 (also known as the Ottaviani Intervention), which the Cardinals Alfredo Ottaviani and Antonio Bacci sent to Paul VI, requesting clarification. This became a rallying point for those who opposed these changes.

In 1971, he became a professor of theology at the Society of St. Pius X's International Seminary at Écône. In 1981, he was consecrated a bishop by Archbishop Ngô Đình Thục, Titular Archbishop of Bulla Regia, a Vietnamese clergyman of the Roman Catholic Church. The Vatican's Sacred Congregation for the Doctrine of the Faith judged in 1983 that he had incurred "ipso facto excommunication" as a consequence. Regardless, Guérard des Lauriers himself later consecrated as bishops two priest who also endorsed sedeprivationism; Günther Storck and Robert McKenna.