Objection 1. It seems that a sacrament does not imprint a character on the soul. For the word "character" seems to signify some kind of distinctive sign. But Christ's members are distinguished from others by eternal predestination, which does not imply anything in the predestined, but only in God predestinating, as we have stated in I, 23, 2. For it is written (2 Timothy 2:19): "The sure foundation of God standeth firm, having this seal: The Lord knoweth who are His." Therefore the sacraments do not imprint a character on the soul.
Objection 2. Further, a character is a distinctive sign. Now a sign, as Augustine says (De Doctr. Christ. ii) "is that which conveys something else to the mind, besides the species which it impresses on the senses." But nothing in the soul can impress a species on the senses. Therefore it seems that no character is imprinted on the soul by the sacraments.
Objection 3. Further, just as the believer is distinguished from the unbeliever by the sacraments of the New Law, so was it under the Old Law. But the sacraments of the Old Law did not imprint a character; whence they are called "justices of the flesh" (Hebrews 9:10) by the Apostle. Therefore neither seemingly do the sacraments of the New Law.
On the contrary, The Apostle says (2 Corinthians 1:21-22): "He... that hath anointed us is God; Who also hath sealed us, and given the pledge of the spirit in our hearts." But a character means nothing else than a kind of sealing. Therefore it seems that by the sacraments God imprints His character on us.
I answer that, As is clear from what has been already stated (62, 5) the sacraments of the New Law are ordained for a twofold purpose; namely, for a remedy against sins; and for the perfecting of the soul in things pertaining to the Divine worship according to the rite of the Christian life. Now whenever anyone is deputed to some definite purpose he is wont to receive some outward sign thereof; thus in olden times soldiers who enlisted in the ranks used to be marked with certain characters on the body, through being deputed to a bodily service. Since, therefore, by the sacraments men are deputed to a spiritual service pertaining to the worship of God, it follows that by their means the faithful receive a certain spiritual character. Wherefore Augustine says (Contra Parmen. ii): "If a deserter from the battle, through dread of the mark of enlistment on his body, throws himself on the emperor's clemency, and having besought and received mercy, return to the fight; is that character renewed, when the man has been set free and reprimanded? is it not rather acknowledged and approved? Are the Christian sacraments, by any chance, of a nature less lasting than this bodily mark?"
Reply to Objection 1. The faithful of Christ are destined to the reward of the glory that is to come, by the seal of Divine Predestination. But they are deputed to acts becoming the Church that is now, by a certain spiritual seal that is set on them, and is called a character.
Reply to Objection 2. The character imprinted on the soul is a kind of sign in so far as it is imprinted by a sensible sacrament: since we know that a certain one has received the baptismal character, through his being cleansed by the sensible water. Nevertheless from a kind of likeness, anything that assimilates one thing to another, or discriminates one thing from another, even though it be not sensible, can be called a character or a seal; thus the Apostle calls Christ "the figure" or charakter "of the substance of the Father" (Hebrews 1:3).
Reply to Objection 3. As stated above (Question 62, Article 6) the sacraments of the Old Law had not in themselves any spiritual power of producing a spiritual effect. Consequently in those sacraments there was no need of a spiritual character, and bodily circumcision sufficed, which the Apostle calls "a seal" (Romans 4:11).
Source: Summa Theologica (New Advent)