Objection 1. It seems that the proper form of this sacrament is not: "I sign thee with the sign of the cross, I confirm thee with the chrism of salvation, in the name of the Father and of the Son and of the Holy Ghost. Amen." For the use of the sacraments is derived from Christ and the apostles. But neither did Christ institute this form, nor do we read of the apostles making use of it. Therefore it is not the proper form of this sacrament.
Objection 2. Further, just as the sacrament is the same everywhere, so should the form be the same: because everything has unity, just as it has being, from its form. But this form is not used by all: for some say: "I confirm thee with the chrism of sanctification." Therefore the above is not the proper form of this sacrament.
Objection 3. Further, this sacrament should be conformed to Baptism, as the perfect to the thing perfected, as stated above (2, Objection 2). But in the form of Baptism no mention is made of signing the character; nor again of the cross of Christ, though in Baptism man dies with Christ, as the Apostle says (Romans 6:3-8); nor of the effect which is salvation, though Baptism is necessary for salvation. Again, in the baptismal form, only one action is included; and the person of the baptizer is expressed in the words: "I baptize thee, whereas the contrary is to be observed in the above form." Therefore this is not the proper form of this sacrament.
On the contrary, Is the authority of the Church, who always uses this form.
I answer that, The above form is appropriate to this sacrament. For just as the form of a natural thing gives it its species, so a sacramental form should contain whatever belongs to the species of the sacrament. Now as is evident from what has been already said (1-2), in this sacrament the Holy Ghost is given for strength in the spiritual combat. Wherefore in this sacrament three things are necessary; and they are contained in the above form. The first of these is the cause conferring fulness of spiritual strength which cause is the Blessed Trinity: and this is expressed in the words, "In the name of the Father," etc. The second is the spiritual strength itself bestowed on man unto salvation by the sacrament of visible matter; and this is referred to in the words, "I confirm thee with the chrism of salvation." The third is the sign which is given to the combatant, as in a bodily combat: thus are soldiers marked with the sign of their leaders. And to this refer the words, "I sign thee with the sign of the cross," in which sign, to wit, our King triumphed (cf. Colossians 2:15).
Reply to Objection 1. As stated above (2, ad 1), sometimes the effect of this sacrament, i.e. the fulness of the Holy Ghost, was given through the ministry of the apostles, under certain visible signs, wrought miraculously by God, Who can bestow the sacramental effect, independently of the sacrament. In these cases there was no need for either the matter or the form of this sacrament. On the other hand, sometimes they bestowed this sacrament as ministers of the sacraments. And then, they used both matter and form according to Christ's command. For the apostles, in conferring the sacraments, observed many things which are not handed down in those Scriptures that are in general use. Hence Dionysius says at the end of his treatise on the Ecclesiastical Hierarchy (chap. vii): "It is not allowed to explain in writing the prayers which are used in the sacraments, and to publish their mystical meaning, or the power which, coming from God, gives them their efficacy; we learn these things by holy tradition without any display,"* i.e. secretly. [The passage quoted in the text of the Summa differs slightly from the above, which is translated directly from the works of Dionysius.] Hence the Apostle, speaking of the celebration of the Eucharist, writes (1 Corinthians 11:34): "The rest I will set in order, when I come."
Reply to Objection 2. Holiness is the cause of salvation. Therefore it comes to the same whether we say "chrism of salvation" or "of sanctification."
Reply to Objection 3. Baptism is the regeneration unto the spiritual life, whereby man lives in himself. And therefore in the baptismal form that action alone is expressed which refers to the man to be sanctified. But this sacrament is ordained not only to the sanctification of man in himself, but also to strengthen him in his outward combat. Consequently not only is mention made of interior sanctification, in the words, "I confirm thee with the chrism of salvation": but furthermore man is signed outwardly, as it were with the standard of the cross, unto the outward spiritual combat; and this is signified by the words, "I sign thee with the sign of the cross."
But in the very word "baptize," which signifies "to cleanse," we can understand both the matter, which is the cleansing water, and the effect, which is salvation. Whereas these are not understood by the word "confirm"; and consequently they had to be expressed.
Again, it has been said above (66, 5, ad 1) that the pronoun "I" is not necessary to the Baptismal form, because it is included in the first person of the verb. It is, however, included in order to express the intention. But this does not seem so necessary in Confirmation, which is conferred only by a minister of excellence, as we shall state later on (11).
Source: Summa Theologica (New Advent)