Stand: Literally, "to hold him"
Objection 1. It seems that he who is confirmed needs no one to stand for him. For this sacrament is given not only to children but also to adults. But adults can stand for themselves. Therefore it is absurd that someone else should stand for them.
Objection 2. Further, he that belongs already to the Church, has free access to the prince of the Church, i.e. the bishop. But this sacrament, as stated above (Article 6), is given only to one that is baptized, who is already a member of the Church. Therefore it seems that he should not be brought by another to the bishop in order to receive this sacrament.
Objection 3. Further, this sacrament is given for spiritual strength, which has more vigor in men than in women, according to Proverbs 31:10: "Who shall find a valiant woman?" Therefore at least a woman should not stand for a man in confirmation.
On the contrary, Are the following words of Pope Innocent, which are to be found in the Decretals (XXX, 4): "If anyone raise the children of another's marriage from the sacred font, or stand for them in Confirmation," etc. Therefore, just as someone is required as sponsor of one who is baptized, so is someone required to stand for him who is to be confirmed.
I answer that, As stated above (1-4-9), this sacrament is given to man for strength in the spiritual combat. Now, just as one newly born requires someone to teach him things pertaining to ordinary conduct, according to Hebrews 12:9: "We have had fathers of our flesh, for instructors, and we obeyed [Vulgate: 'reverenced']" them; so they who are chosen for the fight need instructors by whom they are informed of things concerning the conduct of the battle, and hence in earthly wars, generals and captains are appointed to the command of the others. For this reason he also who receives this sacrament, has someone to stand for him, who, as it were, has to instruct him concerning the fight.
Likewise, since this sacrament bestows on man the perfection of spiritual age, as stated above (2 and 5), therefore he who approaches this sacrament is upheld by another, as being spiritually a weakling and a child.
Reply to Objection 1. Although he who is confirmed, be adult in body, nevertheless he is not yet spiritually adult.
Reply to Objection 2. Though he who is baptized is made a member of the Church, nevertheless he is not yet enrolled as a Christian soldier. And therefore he is brought to the bishop, as to the commander of the army, by one who is already enrolled as a Christian soldier. For one who is not yet confirmed should not stand for another in Confirmation.
Reply to Objection 3. According to Colossians 3 (Galatians 3:28), "in Christ Jesus there is neither male nor female." Consequently it matters not whether a man or a woman stand for one who is to be confirmed.
Source: Summa Theologica (New Advent)