Objection 1. It seems that this sacrament should not be given to man on the forehead. For this sacrament perfects Baptism, as stated above (65, 3-4). But the sacrament of Baptism is given to man over his whole body. Therefore this sacrament should not be given on the forehead only.
Objection 2. Further, this sacrament is given for spiritual strength, as stated above (1-2-4). But spiritual strength is situated principally in the heart. Therefore this sacrament should be given over the heart rather than on the forehead.
Objection 3. Further, this sacrament is given to man that he may freely confess the faith of Christ. But "with the mouth, confession is made unto salvation," according to Romans 10:10. Therefore this sacrament should be given about the mouth rather than on the forehead.
On the contrary, Rabanus says (De Instit. Cleric. i): "The baptized is signed by the priest with chrism on the top of the head, but by the bishop on the forehead."
I answer that, As stated above (1 and 4), in this sacrament man receives the Holy Ghost for strength in the spiritual combat, that he may bravely confess the Faith of Christ even in face of the enemies of that Faith. Wherefore he is fittingly signed with the sign of the cross on the forehead, with chrism, for two reasons. First, because he is signed with the sign of the cross, as a soldier with the sign of his leader, which should be evident and manifest. Now, the forehead, which is hardly ever covered, is the most conspicuous part of the human body. Wherefore the confirmed is anointed with chrism on the forehead, that he may show publicly that he is a Christian: thus too the apostles after receiving the Holy Ghost showed themselves in public, whereas before they remained hidden in the upper room.
Secondly, because man is hindered from freely confessing Christ's name, by two things--by fear and by shame. Now both these things betray themselves principally on the forehead on account of the proximity of the imagination, and because the (vital) spirits mount directly from the heart to the forehead: hence "those who are ashamed, blush, and those who are afraid, pale" (Ethic. iv). And therefore man is signed with chrism, that neither fear nor shame may hinder him from confessing the name of Christ.
Reply to Objection 1. By baptism we are regenerated unto spiritual life, which belongs to the whole man. But in Confirmation we are strengthened for the combat; the sign of which should be borne on the forehead, as in a conspicuous place.
Reply to Objection 2. The principle of fortitude is in the heart, but its sign appears on the forehead: wherefore it is written (Ezekiel 3:8): "Behold I have made... thy forehead harder than their foreheads." Hence the sacrament of the Eucharist, whereby man is confirmed in himself, belongs to the heart, according to Psalm 103:15: "That bread may strengthen man's heart." But the sacrament of Confirmation is required as a sign of fortitude against others; and for this reason it is given on the forehead.
Reply to Objection 3. This sacrament is given that we may confess freely: but not that we may confess simply, for this is also the effect of Baptism. And therefore it should not be given on the mouth, but on the forehead, where appear the signs of those passions which hinder free confession.
Source: Summa Theologica (New Advent)