Objection 1. It seems that the sacramental character is not the character of Christ. For it is written (Ephesians 4:30): "Grieve not the Holy Spirit of God, whereby you are sealed." But a character consists essentially in some. thing that seals. Therefore the sacramental character should be attributed to the Holy Ghost rather than to Christ.
Objection 2. Further, a character has the nature of a sign. And it is a sign of the grace that is conferred by the sacrament. Now grace is poured forth into the soul by the whole Trinity; wherefore it is written (Psalm 83:12): "The Lord will give grace and glory." Therefore it seems that the sacramental character should not be attributed specially to Christ.
Objection 3. Further, a man is marked with a character that he may be distinguishable from others. But the saints are distinguishable from others by charity, which, as Augustine says (De Trin. xv), "alone separates the children of the Kingdom from the children of perdition": wherefore also the children of perdition are said to have "the character of the beast" (Apocalypse 13:16-17). But charity is not attributed to Christ, but rather to the Holy Ghost according to Romans 5:5: "The charity of God is poured forth in our hearts, by the Holy Ghost, Who is given to us"; or even to the Father, according to 2 Corinthians 13:13: "The grace of our Lord Jesus Christ and the charity of God." Therefore it seems that the sacramental character should not be attributed to Christ.
On the contrary, Some define character thus: "A character is a distinctive mark printed in a man's rational soul by the eternal Character, whereby the created trinity is sealed with the likeness of the creating and re-creating Trinity, and distinguishing him from those who are not so enlikened, according to the state of faith." But the eternal Character is Christ Himself, according to Hebrews 1:3: "Who being the brightness of His glory and the figure," or character, "of His substance." It seems, therefore, that the character should properly be attributed to Christ.
I answer that, As has been made clear above (Article 1), a character is properly a kind of seal, whereby something is marked, as being ordained to some particular end: thus a coin is marked for use in exchange of goods, and soldiers are marked with a character as being deputed to military service. Now the faithful are deputed to a twofold end. First and principally to the enjoyment of glory. And for this purpose they are marked with the seal of grace according to Ezekiel 9:4: "Mark Thou upon the foreheads of the men that sigh and mourn"; and Apocalypse 7:3: "Hurt not the earth, nor the sea, nor the trees, till we sign the servants of our God in their foreheads."
Secondly, each of the faithful is deputed to receive, or to bestow on others, things pertaining to the worship of God. And this, properly speaking, is the purpose of the sacramental character. Now the whole rite of the Christian religion is derived from Christ's priesthood. Consequently, it is clear that the sacramental character is specially the character of Christ, to Whose character the faithful are likened by reason of the sacramental characters, which are nothing else than certain participations of Christ's Priesthood, flowing from Christ Himself.
Reply to Objection 1. The Apostle speaks there of that sealing by which a man is assigned to future glory, and which is effected by grace. Now grace is attributed to the Holy Ghost, inasmuch as it is through love that God gives us something gratis, which is the very nature of grace: while the Holy Ghost is love. Wherefore it is written (1 Corinthians 12:4): "There are diversities of graces, but the same Spirit."
Reply to Objection 2. The sacramental character is a thing as regards the exterior sacrament, and a sacrament in regard to the ultimate effect. Consequently, something can be attributed to a character in two ways. First, if the character be considered as a sacrament: and thus it is a sign of the invisible grace which is conferred in the sacrament. Secondly, if it be considered as a character. And thus it is a sign conferring on a man a likeness to some principal person in whom is vested the authority over that to which he is assigned: thus soldiers who are assigned to military service, are marked with their leader's sign, by which they are, in a fashion, likened to him. And in this way those who are deputed to the Christian worship, of which Christ is the author, receive a character by which they are likened to Christ. Consequently, properly speaking, this is Christ's character.
Reply to Objection 3. A character distinguishes one from another, in relation to some particular end, to which he, who receives the character is ordained: as has been stated concerning the military character (1) by which a soldier of the king is distinguished from the enemy's soldier in relation to the battle. In like manner the character of the faithful is that by which the faithful of Christ are distinguished from the servants of the devil, either in relation to eternal life, or in relation to the worship of the Church that now is. Of these the former is the result of charity and grace, as the objection runs; while the latter results from the sacramental character. Wherefore the "character of the beast" may be understood by opposition, to mean either the obstinate malice for which some are assigned to eternal punishment, or the profession of an unlawful form of worship.
Source: Summa Theologica (New Advent)