The Vatican's doctrine office went above and beyond the minimal sanctions foreseen by the church's canon law to respond to the consecrations on Wednesday of four new bishops at the society's Econe, Switzerland, seminary.
The society, known by its acronym SSPX, celebrates the ancient Latin mass and opposes the modernising reforms of the Catholic Church, which it considers to be rife with heresies and errors and has accused of straying from the Catholic faith.
During a ritual-filled, five-hour Mass on Wednesday, attended by some 15,500 people and their children, the SSPX consecrated four new bishops in direct defiance of Pope Leo XIV, who had urged the SSPX to hold off for the sake of the church's unity.
In a decree, the Vatican excommunicated the four new bishops and the two bishops who participated in the ceremony.
It declared the consecrations a "schismatic act" and declared the society itself had created a schism, or intentional rupture with the Catholic Church.
The Vatican warned the faithful who went to the society's masses to stop, declaring "those who adhere formally" to the society are considered themselves schismatic and excommunicated.
It declared SSPX priests to be schismatic, and therefore excommunicated, and invalidated the sacraments of confession and marriage that they administer.
The sanctions were particularly harsh and reversed concessions the Vatican had granted the SSPX in recent years as part of its outreach to bring the group back under Rome's wing.
French Archbishop Marcel Lefebvre founded the SSPX in 1970 in opposition to the modernising reforms of the Second Vatican Council.
Among other things, the 1960s meetings known as Vatican II revolutionised the church's relations with other Christians, Jews and people of other faiths and allowed mass to be celebrated in the vernacular rather than Latin.
Lefebvre consecrated four bishops without papal consent in 1988.
The Vatican promptly excommunicated Lefebvre and the four bishops and declared the consecrations a "schismatic act".
Pope Benedict XVI in 2009 lifted the excommunications as part of his years-long outreach to the group, but the SSPX today has no legal standing in the church and with Thursday's decree is declared to be in schism.
The consecrations had posed a crisis for Leo because the American pontiff has stressed the need for church unity.
He has reached out especially to the conservative and traditionalist wing of the church that was alienated during the Pope Francis pontificate.
But the sanctions imposed on Thursday suggest that after nearly five decades of trying to negotiate with the society, the Holy See has had enough.
The group now has six bishops, 751 priests, 264 seminarians training in five seminaries, 145 religious brothers, 88 oblates and 250 religious sisters representing 50 nationalities, according to SSPX statistics.
The SSPX has accused the church of being rife with errors, such as modernism and liberalism, and that only it is upholding the true faith of Christ.
In his homily during the consecrations on Wednesday, the Reverend Davide Pagliarani, the SSPX superior, insisted the consecrations served Leo and the church.
"We are accused of not respecting the Pope," Pagliarani said.
"But it is precisely because we love the pope as the vicar of Christ, as the head of the church, that we don't want to see the Pope humiliated any more, on the side of false shepherds representing false religions."
