Objection 1. It seems that vainglory is a mortal sin. For nothing precludes the eternal reward except a mortal sin. Now vainglory precludes the eternal reward: for it is written (Matthew 6:1): "Take heed, that you do not give justice before men, to be seen by them: otherwise you shall not have a reward of your Father Who is in heaven." Therefore vainglory is a mortal sin.
Objection 2. Further, whoever appropriates to himself that which is proper to God, sins mortally. Now by desiring vainglory, a man appropriates to himself that which is proper to God. For it is written (Isaiah 42:8): "I will not give My glory to another," and (1 Timothy 1:17): "To... the only God be honor and glory." Therefore vainglory is a mortal sin.
Objection 3. Further, apparently a sin is mortal if it be most dangerous and harmful. Now vainglory is a sin of this kind, because a gloss of Augustine on 1 Thessalonians 2:4, "God, Who proveth our hearts," says: "Unless a man war against the love of human glory he does not perceive its baneful power, for though it be easy for anyone not to desire praise as long as one does not get it, it is difficult not to take pleasure in it, when it is given." Chrysostom also says (Hom. xix in Matth.) that "vainglory enters secretly, and robs us insensibly of all our inward possessions." Therefore vainglory is a mortal sin.
On the contrary, Chrysostom says [Hom. xiii in the Opus Imperfectum falsely ascribed to St. John Chrysostom] that "while other vices find their abode in the servants of the devil, vainglory finds a place even in the servants of Christ." Yet in the latter there is no mortal sin. Therefore vainglory is not a mortal sin.
I answer that, As stated above (24, 12; 110, 4; 112, 2), a sin is mortal through being contrary to charity. Now the sin of vainglory, considered in itself, does not seem to be contrary to charity as regards the love of one's neighbor: yet as regards the love of God it may be contrary to charity in two ways. On one way, by reason of the matter about which one glories: for instance when one glories in something false that is opposed to the reverence we owe God, according to Ezekiel 28:2, "Thy heart is lifted up, and Thou hast said: I am God," and 1 Corinthians 4:7, "What hast thou that thou hast not received? And if thou hast received, why dost thou glory, as if thou hadst not received it?" Or again when a man prefers to God the temporal good in which he glories: for this is forbidden (Jeremiah 9:23-24): "Let not the wise man glory in his wisdom, and let not the strong man glory in his strength, and let not the rich man glory in his riches. But let him that glorieth glory in this, that he understandeth and knoweth Me." Or again when a man prefers the testimony of man to God's; thus it is written in reproval of certain people (John 12:43): "For they loved the glory of men more than the glory of God."
In another way vainglory may be contrary to charity, on the part of the one who glories, in that he refers his intention to glory as his last end: so that he directs even virtuous deeds thereto, and, in order to obtain it, forbears not from doing even that which is against God. On this way it is a mortal sin. Wherefore Augustine says (De Civ. Dei v, 14) that "this vice," namely the love of human praise, "is so hostile to a godly faith, if the heart desires glory more than it fears or loves God, that our Lord said (John 5:44): How can you believe, who receive glory one from another, and the glory which is from God alone, you do not seek?"
If, however, the love of human glory, though it be vain, be not inconsistent with charity, neither as regards the matter gloried in, nor as to the intention of him that seeks glory, it is not a mortal but a venial sin.
Reply to Objection 1. No man, by sinning, merits eternal life: wherefore a virtuous deed loses its power to merit eternal life, if it be done for the sake of vainglory, even though that vainglory be not a mortal sin. On the other hand when a man loses the eternal reward simply through vainglory, and not merely in respect of one act, vainglory is a mortal sin.
Reply to Objection 2. Not every man that is desirous of vainglory, desires the excellence which belongs to God alone. For the glory due to God alone differs from the glory due to a virtuous or rich man.
Reply to Objection 3. Vainglory is stated to be a dangerous sin, not only on account of its gravity, but also because it is a disposition to grave sins, in so far as it renders man presumptuous and too self-confident: and so it gradually disposes a man to lose his inward goods.
Source: Summa Theologica (New Advent)