Pride is the beginning of sin, the first impulse and movement toward evil. Perhaps indeed it is both the root and the foundation. For "the beginning" means either the first impulse towards evil, or the grounding. ...So also pride is the beginning of sin. For every sin begins from it, and is maintained by it. For that, whatever good things we do, this vice suffers them not to remain and not fall away, but is as a certain root not letting them abide unshaken, is manifest from hence: see what things the Pharisee did, but they profited him nothing. For he did not extirpate the root, but it corrupted all his performances, because the root remained. From pride springs contempt of the poor, desire of riches, the love of power, the longing for much glory. Such an one is prompt to revenge an insult. For he who is proud cannot bear to be insulted even by his superiors, much less by his inferiors. But he who cannot bear to be insulted cannot bear either to suffer any ill. See how pride is the beginning of sin.
This I is one of the best explanations of this vice for I also believe that it is this pride which is the root of heresy, and the reason why there are heretics. Heretics, full of pride, do not humble themselves to rightful ecclesiastical authorities, but declare them to be the heretics, as is the excuse of every heretic throughout history, they have all claimed to be the ones keeping with orthodoxy while, they believe, the Church has embraced the heresy; thus the section on superiors mentioned. Pride is the enemy of humble obedience, a virtue of faith, which keeps a man in the right doctrine, that he submit himself to the rightful authority of the Church in issues of the faith. This pride is also the reason why virtue is seldom found amongst heretics as they, being full of pride as they are, and by this I speak of obstinate heretics, live according to themselves and not according to God, just as they reckon according to themselves and not according to God and His Church.