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Schism is the "refusal of submission to the Supreme (Roman) Pontiff or of communion with the members of the Church subject to him," as defined in canon 751 of the Code of Canon Law and paragraph 2089 of the Catechism of the Catholic Church. Refusal of submission isn't a simple act of disobedience, but rather a denial of the pope's God-given authority to command. Refusal of communion means deliberately refusing to participate in Catholic life and worship. Canon 1364 states that schismatics are automatically excommunicated.
Christ not only bestowed on St. Peter and his successors the charismatic grace of infallibility to teach but also established their papal primacy to authoritatively govern as Christ. Matthew 16:19 declares, "I will give thee the keys of the kingdom of heaven; and whatever thou shalt bind on earth shall be bound in heaven, and whatever thou shalt loose on earth shall be loosed in heaven."
In 1302 Pope Boniface VIII referenced this passage when speaking of the grave necessity of Catholics to submit to papal authority. In his bull Unam Sanctam, Boniface VIII wrote,
This authority ... is not human but rather divine, granted to Peter by a divine word and reaffirmed to him (Peter) and his successors. ... Therefore whoever resists this power thus ordained by God, resists the ordinance of God [Rom 13:2]. ... Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff.
In 1870 Vatican I similarly taught,
Both clergy and faithful, of whatever rite and dignity ... are bound to submit to this power ... not only in matters concerning faith and morals, but also in those which regard the discipline and government of the church throughout the world. ... This is the teaching of the catholic truth, and no one can depart from it without endangering his faith and salvation.
Take a look at how so-called traditionalists rationalize their schism in Church Militant's Premium show FBI—Catholi-Schism.