Pope Leo XIV issued an urgent and final appeal on Tuesday to the archconservative Priestly Society of Saint Pius X not to proceed with the appointment of bishops without papal approval.
The society, also known as the Lefebvrists, is set to consecrate four new bishops in Écône in Switzerland on Wednesday.
Leo said he was addressing the society with a paternal heart and aware of his responsibilities. "In this spirit, and filled with Christian affection, I plead with you and ask you with all my heart: please turn back!" he wrote.
He termed the consecrations a "schismatic act" that would invalidate the sacraments administered to Catholic faithful by the bishops, who are seen as illegitimate in the eyes of the Vatican.
The Vatican appealed to the society four weeks ago, warning that if it proceeded, this would result in excommunication of the priests involved, implying complete exclusion from the Catholic Church.
In the letter, compiled in French, Leo addressed the head of the society, Father Davide Pagliarani. "I pray for you, because to tear the seamless garment of Christ is a sin of extreme gravity," Leo wrote. "May the Lord enlighten your consciences and awaken your hearts," he added.
But Leo also stressed that the church was open to dialogue. "With a sorrowful yet hopeful heart, I feel it is my duty, through the authority received from Christ, to ask you to desist from your intended act," he wrote.
The society, founded in 1970 by French Archbishop Marcel Lefebvre, has made its intentions clear to proceed with the consecrations. It rejects the reforms of the Second Vatican Council held between 1962 and 1965.
In 1988, Lefebvre consecrated four bishops in defiance of John Paul II, the pope at the time. Lefebvre and the four were promptly excommunicated. John Paul II's successor, Benedict XVI, lifted the excommunication in 2009 in an attempt at reconciliation.
