The sin of pride - more grievous than sins of the flesh.

THE DEFECTS BORN OF PRIDE

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The principal defects springing from pride are presumption, ambition, and vainglory.
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Presumption is the desire and inordinate hope of doing what is above one's power.(16) The presumptuous man believes himself capable of studying and solving the most difficult questions; he settles the most abstruse problems with rash haste. He fancies that he has sufficient light to guide himself without consulting a director. Instead of building his interior life on humility, renunciation, fidelity to the duty of the present moment even in little things, he speaks particularly of magnanimity, of apostolic zeal, or indeed aspires to the immediate attainment of the high degrees of prayer without passing through the various stages, forgetting that he is still only a beginner, whose will is still weak and full of egoism. He is still full of self; a great void must be created in him in order that his soul may some day be filled with God and able to give Him to others.
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From presumption springs ambition, under one form or another. Because a man presumes too greatly on his powers and judges himself superior to others, he wishes to dominate them, to impose on them his ideas in matters of doctrine, or to govern them. St. Thomas (17) says that a man manifests ambition when he seeks offices carrying with them honor which he does not merit; when he seeks honors for himself and not for the glory of God or the profit of others. How many schemes, secret solicitations, and intrigues ambition inspires in all walks of life! (18)
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Pride leads also to vainglory, that is, the wish to be esteemed for oneself, without referring this honor to God, the source of all good, and often a wish to be esteemed for vain things. This is the case of the pedant who loves to display his knowledge, binding himself and wishing to bind others to trifles.(19)
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Defects from vainglory.
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Many defects spring from vainglory: (20) boasting, which easily makes a person ridiculous; hypocrisy, which under the appearances of virtue, hides vices; stubbornness, contention or asperity in defending one's opinion, which engenders discord; and also disobedience, sharp criticisms of superiors.
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The way of perdition.
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Thus we see that pride which is not repressed sometimes produces disastrous effects. How many discords, hatreds, and wars are born of pride! It has been justly said that pride is the great enemy of perfection because it is the source of numerous sins and deprives us of many graces and merits. Scripture says: "God resisteth the proud and giveth grace to the humble." (21) And Christ says of the Pharisees, who pray and give alms in order to be seen by men: "They have received their reward"; (22) they cannot expect that of our heavenly Father, since they have acted for themselves and not for Him. Lastly, a life dominated by pride is grievously sterile and presages perdition unless a remedy is promptly applied. - Garrigou-Lagrange