Joaquín Sáenz y Arriaga

Joaquín Sáenz y Arriaga (12 October 1899 – 28 April 1976) was a Mexican Catholic priest and theologian who was a member of the Society of Jesus but who was excommunicated from the Church. He played a significant role in contemporary traditionalist Catholicism, both in his native Mexico and in the wider Catholic world, as a pioneering theorist of sedevacantism, claiming that Paul VI (Giovanni Montini) was a heretical Antipope and thus not a legitimate Pope of the Catholic Church. His two most prominent works, The New Montinian Church (1971) and Sede Vacante (1973) have been described as "foundational" in the development of sedevacantism.

As a member of the Jesuits, having developed as a seminarian during the times of the Cristero War in the aftermath of the Mexican Revolution and state persecution of the Catholic Church in Mexico, Sáenz y Arriaga closely involved himself in the struggle for Catholic education in Mexico. This brought him into close association with the secret society Los TECOS, associated with the Universidad Autónoma de Guadalajara from the 1930s onward. This Mexican secret society of which he was the spiritual advisor, endorsed Catholic integralism, was staunchly anti-communist, concerning itself with endorsing what it claimed was "Christian order" against the so-called "Judeo-Masonic conspiracy."

Sáenz y Arriaga was directly involved in lobbying at the Second Vatican Council against the development of the document Nostra aetate, which dealt with relations between the Catholic Church and the Jews. In the aftermath of the council, he was involved in public controversy against the pioneers of Latin American liberation theology, which attempted to create a synthesis of Christianity and Marxism: aimed particularly at Sergio Méndez Arceo, the Bishop of Cuernavaca and his CIDOC in Mexico, but also in a wider context, the Second Conference of CELAM at Medellín, which he attended. In the later years of his life, he was ever more publicly critical of Paul VI, finally declaring him in published works a heretical non-Catholic Antipope, for which he was excommunicated by Cardinal Miguel Darío Miranda y Gómez in 1971. In liturgy, he completely rejected the New Order of Mass and continued to celebrate the Traditional Latin Mass. His publication, Trento, which he operated with Fr. Moisés Carmona, became the basis of the Unión Católica Trento and organised sedevacantism in Mexico and later internationally.