Objection 1. It seems that the sacrament of Confirmation does not imprint a character. For a character means a distinctive sign. But a man is not distinguished from unbelievers by the sacrament of Confirmation, for this is the effect of Baptism; nor from the rest of the faithful, because this sacrament is ordained to the spiritual combat, which is enjoined to all the faithful. Therefore a character is not imprinted in this sacrament.
Objection 2. Further, it was stated above (Question 63, Article 2) that a character is a spiritual power. Now a power must be either active or passive. But the active power in the sacraments is conferred by the sacrament of order: while the passive or receptive power is conferred by the sacrament of Baptism. Therefore no character is imprinted by the sacrament of Confirmation.
Objection 3. Further, in circumcision, which is a character of the body, no spiritual character is imprinted. But in this sacrament a character is imprinted on the body, when the sign of the cross is signed with chrism on man's brow. Therefore a spiritual character is not imprinted by this sacrament.
On the contrary, A character is imprinted in every sacrament that is not repeated. But this sacrament is not repeated: for Gregory II says (Ep. iv ad Bonifac.): "As to the man who was confirmed a second time by a bishop, such a repetition must be forbidden." Therefore a character is imprinted in Confirmation.
I answer that, As stated above (Question 63, Article 2), a character is a spiritual power ordained to certain sacred actions. Now it has been said above (1; 65, 1) that, just as Baptism is a spiritual regeneration unto Christian life, so also is Confirmation a certain spiritual growth bringing man to perfect spiritual age. But it is evident, from a comparison with the life of the body, that the action which is proper to man immediately after birth, is different from the action which is proper to him when he has come to perfect age. And therefore by the sacrament of Confirmation man is given a spiritual power in respect of sacred actions other than those in respect of which he receives power in Baptism. For in Baptism he receives power to do those things which pertain to his own salvation, forasmuch as he lives to himself: whereas in Confirmation he receives power to do those things which pertain to the spiritual combat with the enemies of the Faith. This is evident from the example of the apostles, who, before they received the fulness of the Holy Ghost, were in the "upper room... persevering... in prayer" (Acts 1:13-14); whereas afterwards they went out and feared not to confess their faith in public, even in the face of the enemies of the Christian Faith. And therefore it is evident that a character is imprinted in the sacrament of Confirmation.
Reply to Objection 1. All have to wage the spiritual combat with our invisible enemies. But to fight against visible foes, viz. against the persecutors of the Faith, by confessing Christ's name, belongs to the confirmed, who have already come spiritually to the age of virility, according to 1 John 2:14: "I write unto you, young men, because you are strong, and the word of God abideth in you, and you have overcome the wicked one." And therefore the character of Confirmation is a distinctive sign, not between unbelievers and believers, but between those who are grown up spiritually and those of whom it is written: "As new-born babes" (1 Peter 2:2).
Reply to Objection 2. All the sacraments are protestations of faith. Therefore just as he who is baptized receives the power of testifying to his faith by receiving the other sacraments; so he who is confirmed receives the power of publicly confessing his faith by words, as it were "ex officio."
Reply to Objection 3. The sacraments of the Old Law are called "justice of the flesh" (Hebrews 9:10) because, to wit, they wrought nothing inwardly. Consequently in circumcision a character was imprinted in the body only, but not in the soul. But in Confirmation, since it is a sacrament of the New Law, a spiritual character is imprinted at the same time, together with the bodily character.
Source: Summa Theologica (New Advent)