Objection 1. It would seem that this sacrament has no form. Because, since the efficacy of the sacraments is derived from their institution, as also from their form, the latter must needs be appointed by the institutor of the sacrament. But there is no account of the form of this sacrament being instituted either by Christ or by the apostles. Therefore this sacrament has no form.
Objection 2. Further, whatever is essential to a sacrament is observed everywhere in the same way. Now nothing is so essential to a sacrament that has a form, as that very form. Therefore, as in this sacrament there is no form commonly used by all, since various words are in use, it seems that this sacrament has no form.
Objection 3. Further, in Baptism no form is needed except for the sanctification of the matter, because the water is "sanctified by the word of life so as to wash sin away," as Hugh states (De Sacram. ii). Now the matter of this sacrament is already consecrated. Therefore it needs no form of words.
On the contrary, The Master says (Sent. iv, D, 1) that every sacrament of the New Law consists in things and words. Now the words are the sacramental form. Therefore, since this is a sacrament of the New Law, it seems that it has a form.
Further, this is confirmed by the rite of the Universal Church, who uses certain words in the bestowal of this sacrament.
I answer that, Some have held that no form is essential to this sacrament. This, however, seems derogatory to the effect of this sacrament, since every sacrament signifies its effect. Now the matter is indifferent as regards its effect, and consequently cannot be determined to any particular effect save by the form of words. Hence in all the sacraments of the New Law, since they effect what they signify, there must needs be things and words. Moreover James (5:14-15) seems to ascribe the whole force of this sacrament to prayer, which is the form thereof, as we shall state further on (ad 2: 8,9). Wherefore the foregoing opinion seems presumptuous and erroneous; and for that reason we should hold with the common opinion that this, like all the other sacraments, has a fixed form.
Reply to Objection 1. Holy Writ is proposed to all alike: and so, the form of Baptism, which can be conferred by all, should be expressed in Holy Writ, as also the form of the Eucharist, which in regard to that sacrament, expresses faith which is necessary for salvation. Now the forms of the other sacraments are not contained in Holy Writ, but were handed down to the Church by the apostles, who received them from our Lord, as the Apostle declares (1 Corinthians 11:23): "For I have received of the Lord that which also I delivered to you," etc.
Reply to Objection 2. The words which are essential to the form, viz. the prayer of deprecation, are said by all; but other words which pertain to the well-being thereof, are not said by all.
Reply to Objection 3. The matter of Baptism has a certain sanctification of its own from the very contact of our Saviour's flesh; but the form of words sanctifies it so that it has a sanctifying force. In like manner when the matter of this sacrament has been sanctified in itself, it requires sanctification in its use, so that it may sanctify actually.
Source: Summa Theologica (New Advent)