Objection 1. It seems that a character is not a spiritual power. For "character" seems to be the same thing as "figure"; hence (Hebrews 1:3), where we read "figure of His substance, "for "figure" the Greek has charakter. Now "figure" is in the fourth species of quality, and thus differs from power which is in the second species. Therefore character is not a spiritual power.
Objection 2. Further, Dionysius says (Eccl. Hier. ii): "The Divine Beatitude admits him that seeks happiness to a share in Itself, and grants this share to him by conferring on him Its light as a kind of seal." Consequently, it seems that a character is a kind of light. Now light belongs rather to the third species of quality. Therefore a character is not a power, since this seems to belong to the second species.
Objection 3. Further, character is defined by some thus: "A character is a holy sign of the communion of faith and of the holy ordination conferred by a hierarch." Now a sign is in the genus of "relation," not of "power." Therefore a character is not a spiritual power.
Objection 4. Further, a power is in the nature of a cause and principle (Metaph. v). But a "sign" which is set down in the definition of a character is rather in the nature of an effect. Therefore a character is not a spiritual power.
On the contrary, The Philosopher says (Ethic. ii): "There are three things in the soul, power, habit, and passion." Now a character is not a passion: since a passion passes quickly, whereas a character is indelible, as will be made clear further on (5). In like manner it is not a habit: because no habit is indifferent to acting well or ill: whereas a character is indifferent to either, since some use it well, some ill. Now this cannot occur with a habit: because no one abuses a habit of virtue, or uses well an evil habit. It remains, therefore, that a character is a power.
I answer that, As stated above (Article 1), the sacraments of the New Law produce a character, in so far as by them we are deputed to the worship of God according to the rite of the Christian religion. Wherefore Dionysius (Eccl. Hier. ii), after saying that God "by a kind of sign grants a share of Himself to those that approach Him," adds "by making them Godlike and communicators of Divine gifts." Now the worship of God consists either in receiving Divine gifts, or in bestowing them on others. And for both these purposes some power is needed; for to bestow something on others, active power is necessary; and in order to receive, we need a passive power. Consequently, a character signifies a certain spiritual power ordained unto things pertaining to the Divine worship.
But it must be observed that this spiritual power is instrumental: as we have stated above (Question 62, Article 4) of the virtue which is in the sacraments. For to have a sacramental character belongs to God's ministers: and a minister is a kind of instrument, as the Philosopher says (Polit. i). Consequently, just as the virtue which is in the sacraments is not of itself in a genus, but is reducible to a genus, for the reason that it is of a transitory and incomplete nature: so also a character is not properly in a genus or species, but is reducible to the second species of quality.
Reply to Objection 1. Configuration is a certain boundary of quantity. Wherefore, properly speaking, it is only in corporeal things; and of spiritual things is said metaphorically. Now that which decides the genus or species of a thing must needs be predicated of it properly. Consequently, a character cannot be in the fourth species of quality, although some have held this to be the case.
Reply to Objection 2. The third species of quality contains only sensible passions or sensible qualities. Now a character is not a sensible light. Consequently, it is not in the third species of quality as some have maintained.
Reply to Objection 3. The relation signified by the word "sign" must needs have some foundation. Now the relation signified by this sign which is a character, cannot be founded immediately on the essence of the soul: because then it would belong to every soul naturally. Consequently, there must be something in the soul on which such a relation is founded. And it is in this that a character essentially consists. Therefore it need not be in the genus "relation" as some have held.
Reply to Objection 4. A character is in the nature of a sign in comparison to the sensible sacrament by which it is imprinted. But considered in itself, it is in the nature of a principle, in the way already explained.
Source: Summa Theologica (New Advent)