Objection 1. It would seem that charity is not lost through one mortal sin. For Origen says (Peri Archon i): "When a man who has mounted to the stage of perfection, is satiated, I do not think that he will become empty or fall away suddenly; but he must needs do so gradually and by little and little." But man falls away by losing charity. Therefore charity is not lost through only one mortal sin.
Objection 2. Further, Pope Leo in a sermon on the Passion (60) addresses Peter thus: "Our Lord saw in thee not a conquered faith, not an averted love, but constancy shaken. Tears abounded where love never failed, and the words uttered in trepidation were washed away by the fount of charity." From this Bernard [William of St. Thierry, De Nat. et Dig. Amoris. vi.] drew his assertion that "charity in Peter was not quenched, but cooled."
But Peter sinned mortally in denying Christ. Therefore charity is not lost through one mortal sin.
Objection 3. Further, charity is stronger than an acquired virtue. Now a habit of acquired virtue is not destroyed by one contrary sinful act. Much less, therefore, is charity destroyed by one contrary mortal sin.
Objection 4. Further, charity denotes love of God and our neighbor. Now, seemingly, one may commit a mortal sin, and yet retain the love of God and one's neighbor; because an inordinate affection for things directed to the end, does not remove the love for the end, as stated above (Article 10). Therefore charity towards God can endure, though there be a mortal sin through an inordinate affection for some temporal good.
Objection 5. Further, the object of a theological virtue is the last end. Now the other theological virtues, namely faith and hope, are not done away by one mortal sin, in fact they remain though lifeless. Therefore charity can remain without a form, even when a mortal sin has been committed.
On the contrary, By mortal sin man becomes deserving of eternal death, according to Romans 6:23: "The wages of sin is death." On the other hand whoever has charity is deserving of eternal life, for it is written (John 14:21): "He that loveth Me, shall be loved by My Father: and I will love Him, and will manifest Myself to him," in which manifestation everlasting life consists, according to John 17:3: "This is eternal life; that they may know Thee the... true God, and Jesus Christ Whom Thou hast sent." Now no man can be worthy, at the same time, of eternal life and of eternal death. Therefore it is impossible for a man to have charity with a mortal sin. Therefore charity is destroyed by one mortal sin.
I answer that, That one contrary is removed by the other contrary supervening. Now every mortal sin is contrary to charity by its very nature, which consists in man's loving God above all things, and subjecting himself to Him entirely, by referring all that is his to God. It is therefore essential to charity that man should so love God as to wish to submit to Him in all things, and always to follow the rule of His commandments; since whatever is contrary to His commandments is manifestly contrary to charity, and therefore by its very nature is capable of destroying charity.
If indeed charity were an acquired habit dependent on the power of its subject, it would not necessarily be removed by one mortal sin, for act is directly contrary, not to habit but to act. Now the endurance of a habit in its subject does not require the endurance of its act, so that when a contrary act supervenes the acquired habit is not at once done away. But charity, being an infused habit, depends on the action of God Who infuses it, Who stands in relation to the infusion and safekeeping of charity, as the sun does to the diffusion of light in the air, as stated above (10, Objection 3). Consequently, just as the light would cease at once in the air, were an obstacle placed to its being lit up by the sun, even so charity ceases at once to be in the soul through the placing of an obstacle to the outpouring of charity by God into the soul.
Now it is evident that through every mortal sin which is contrary to God's commandments, an obstacle is placed to the outpouring of charity, since from the very fact that a man chooses to prefer sin to God's friendship, which requires that we should obey His will, it follows that the habit of charity is lost at once through one mortal sin. Hence Augustine says (Gen. ad lit. viii, 12) that "man is enlightened by God's presence, but he is darkened at once by God's absence, because distance from Him is effected not by change of place but by aversion of the will."
Reply to Objection 1. This saying of Origen may be understood, in one way, that a man who is in the state of perfection, does not suddenly go so far as to commit a mortal sin, but is disposed thereto by some previous negligence, for which reason venial sins are said to be dispositions to mortal sin, as stated above (I-II, 88, 3). Nevertheless he falls, and loses charity through the one mortal sin if he commits it.
Since, however, he adds: "If some slight slip should occur, and he recover himself quickly he does not appear to fall altogether," we may reply in another way, that when he speaks of a man being emptied and falling away altogether, he means one who falls so as to sin through malice; and this does not occur in a perfect man all at once.
Reply to Objection 2. Charity may be lost in two ways; first, directly, by actual contempt, and, in this way, Peter did not lose charity. Secondly, indirectly, when a sin is committed against charity, through some passion of desire or fear; it was by sinning against charity in this way, that Peter lost charity; yet he soon recovered it.
The Reply to the Third Objection is evident from what has been said.
Reply to Objection 4. Not every inordinate affection for things directed to the end, i.e., for created goods, constitutes a mortal sin, but only such as is directly contrary to the Divine will; and then the inordinate affection is contrary to charity, as stated.
Reply to Objection 5. Charity denotes union with God, whereas faith and hope do not. Now every mortal sin consists in aversion from God, as stated above (Gen. ad lit. viii, 12). Consequently every moral sin is contrary to charity, but not to faith and hope, but only certain determinate sins, which destroy the habit of faith or of hope, even as charity is destroyed by every moral sin. Hence it is evident that charity cannot remain lifeless, since it is itself the ultimate form regarding God under the aspect of last end as stated above (Question 23, Article 8).
Source: Summa Theologica (New Advent)