Objection 1. It seems that a character can be blotted out from the soul. Because the more perfect an accident is, the more firmly does it adhere to its subject. But grace is more perfect than a character; because a character is ordained unto grace as to a further end. Now grace is lost through sin. Much more, therefore, is a character so lost.
Objection 2. Further, by a character a man is deputed to the Divine worship, as stated above (3,4). But some pass from the worship of God to a contrary worship by apostasy from the faith. It seems, therefore, that such lose the sacramental character.
Objection 3. Further, when the end ceases, the means to the end should cease also: thus after the resurrection there will be no marriage, because begetting will cease, which is the purpose of marriage. Now the exterior worship to which a character is ordained, will not endure in heaven, where there will be no shadows, but all will be truth without a veil. Therefore the sacramental character does not last in the soul for ever: and consequently it can be blotted out.
On the contrary, Augustine says (Contra Parmen. ii): "The Christian sacraments are not less lasting than the bodily mark" of military service. But the character of military service is not repeated, but is "recognized and approved" in the man who obtains the emperor's forgiveness after offending him. Therefore neither can the sacramental character be blotted out.
I answer that, As stated above (Article 3), in a sacramental character Christ's faithful have a share in His Priesthood; in the sense that as Christ has the full power of a spiritual priesthood, so His faithful are likened to Him by sharing a certain spiritual power with regard to the sacraments and to things pertaining to the Divine worship. For this reason it is unbecoming that Christ should have a character: but His Priesthood is compared to a character, as that which is complete and perfect is compared to some participation of itself. Now Christ's Priesthood is eternal, according to Psalm 109:4: "Thou art a priest for ever, according to the order of Melchisedech." Consequently, every sanctification wrought by His Priesthood, is perpetual, enduring as long as the thing sanctified endures. This is clear even in inanimate things; for the consecration of a church or an altar lasts for ever unless they be destroyed. Since, therefore, the subject of a character is the soul as to its intellective part, where faith resides, as stated above (4, ad 3); it is clear that, the intellect being perpetual and incorruptible, a character cannot be blotted out from the soul.
Reply to Objection 1. Both grace and character are in the soul, but in different ways. For grace is in the soul, as a form having complete existence therein: whereas a character is in the soul, as an instrumental power, as stated above (Article 2). Now a complete form is in its subject according to the condition of the subject. And since the soul as long as it is a wayfarer is changeable in respect of the free-will, it results that grace is in the soul in a changeable manner. But an instrumental power follows rather the condition of the principal agent: and consequently a character exists in the soul in an indelible manner, not from any perfection of its own, but from the perfection of Christ's Priesthood, from which the character flows like an instrumental power.
Reply to Objection 2. As Augustine says (Contra Parmen. ii), "even apostates are not deprived of their baptism, for when they repent and return to the fold they do not receive it again; whence we conclude that it cannot be lost." The reason of this is that a character is an instrumental power, as stated above (ad 1), and the nature of an instrument as such is to be moved by another, but not to move itself; this belongs to the will. Consequently, however much the will be moved in the contrary direction, the character is not removed, by reason of the immobility of the principal mover.
Reply to Objection 3. Although external worship does not last after this life, yet its end remains. Consequently, after this life the character remains, both in the good as adding to their glory, and in the wicked as increasing their shame: just as the character of the military service remains in the soldiers after the victory, as the boast of the conquerors, and the disgrace of the conquered.
Source: Summa Theologica (New Advent)