“Pride is the excessive love of one’s own excellence. It is ordinarily accounted one of the seven capital sins.”
“Pride is the queen of sin. St. Gregory the Great warns us: “For when pride, the queen of sins, has fully possessed a conquered heart, she surrenders it immediately to seven principal sins, as if to some of her generals, to lay it waste” (Moralia 87). Yet what are these seven principal sins that pride invites into the conquered heart? They are, according to Gregory, “vainglory, envy, anger, melancholy, avarice, gluttony, [and] lust.” They are the “first progeny” of pride, the offshoots of its “poisonous root.” As both Gregory and St. Thomas Aquinas note, Scripture teaches: “For pride is the beginning of all sin” (Sir. 10:15, DRA).”
“Aquinas, citing St. Isidore, teaches: “A man is said to be proud, because he wishes to appear above what he really is” (II-II.162.1). Aquinas comments that a man who uses his reason rightly acts “proportionate to him,” but pride causes a man to have a disproportionate understanding of who he truly is. Therefore, the self-understanding of the prideful man is contrary to his reason and sinful (CCC 1849). It is here we may start to see how pride opens the soul to a host of sins.”
Catholic Answers
https://www.catholic.com/magazine/online-edition/knowing-and-fighting-the-queen-of-sin
Aquinas holds that pride is a mortal sin (II-II.162.5). He explains, “The root of pride is found to consist in man not being, in some way, subject to God and his rule,” and “it is evident that not to be subject to God is of its very nature a mortal sin.” It is in fact this unwillingness in man to submit to God and his rule that makes pride “the most grievous of sins” (II-II.162.6).”
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