Objection 1. It would seem that the character is not imprinted on the priest at the moment when the chalice is handed to him. For the consecration of a priest is done by anointing as in Confirmation. Now in Confirmation the character is imprinted at the moment of anointing; and therefore in the priesthood also and not at the handing of the chalice.
Objection 2. Further, our Lord gave His disciples the priestly power when He said (John 20:22-23): "Receive ye the Holy Ghost: whose sins you shall forgive," etc. Now the Holy Ghost is given by the imposition of hands. Therefore the character of order is given at the moment of the imposition of hands.
Objection 3. Further, as the ministers are consecrated, even so are the ministers' vestments. Now the blessing alone consecrates the vestments. Therefore the consecration of the priest also is effected by the mere blessing of the bishop.
Objection 4. Further, as a chalice is handed to the priest, even so is the priestly vestment. Therefore if a character is imprinted at the giving of the chalice, so likewise is there at the giving of the chasuble, and thus a priest would have two characters: but this is false.
Objection 5. Further, the deacon's order is more closely allied to the priest's Order than is the subdeacon's. But if a character is imprinted on the priest at the moment of the handing of the chalice, the subdeacon would be more closely allied to the priest than the deacon; because the subdeacon receives the character at the handing of the chalice and not the deacon. Therefore the priestly character is not imprinted at the handing of the chalice.
Objection 6. Further, the Order of acolytes approaches nearer to the priestly act by exercising an act over the cruet than by exercising an act over the torch. Yet the character is imprinted on the acolytes when they receive the torch rather than when they receive the cruet, because the name of acolyte signifies candle-bearer. Therefore the character is not imprinted on the priest when he receives the chalice.
On the contrary, The principal act of the priest's Order is to consecrate Christ's body. Now he receives the power to this effect at the handing of the chalice. Therefore the character is imprinted on him then.
I answer that, As stated above (4, ad 1), to cause the form and to give the matter its proximate preparation for the form belong to the same agent. Wherefore the bishop in conferring orders does two things; for he prepares the candidates for the reception of orders, and delivers to them the power of order. He prepares them, both by instructing them in their respective offices and by doing something to them, so that they may be adapted to receive the power. This preparation consists of three things, namely blessing, imposition of hands, and anointing. By the blessing they are enlisted in the Divine service, wherefore the blessing is given to all. By the imposition of hands the fulness of grace is given, whereby they are qualified for exalted duties, wherefore only deacons and priests receive the imposition of hands, because they are competent to dispense the sacraments, although the latter as principal dispensers, the former as ministers. But by the anointing they are consecrated for the purpose of handling the sacrament, wherefore the anointing is done to the priests alone who touch the body of Christ with their own hands; even as a chalice is anointed because it holds the blood, and the paten because it holds the body.
The conferring of power is effected by giving them something pertaining to their proper act. And since the principal act of a priest is to consecrate the body and blood of Christ, the priestly character is imprinted at the very giving of the chalice under the prescribed form of words.
Reply to Objection 1. In Confirmation there is not given the office of exercising an act on an exterior matter, wherefore the character is not imprinted in that sacrament at the handing of some particular thing, but at the mere imposition of hands and anointing. But it is otherwise in the priestly Order, and consequently the comparison fails.
Reply to Objection 2. Our Lord gave His disciples the priestly power, as regards the principal act, before His passion at the supper when He said: "Take ye and eat" (Matthew 26:26), wherefore He added: "Do this for a commemoration of Me" (Luke 22:19). After the resurrection, however, He gave them the priestly power, as to its secondary act, which is to bind and loose.
Reply to Objection 3. Vestments require no other consecration except to be set aside for the Divine worship, wherefore the blessing suffices for their consecration. But it is different with those who are ordained, as explained above.
Reply to Objection 4. The priestly vestment signifies, not the power given to the priest, but the aptitude required of him for exercising the act of that power. Wherefore a character is imprinted neither on the priest nor on anyone else at the giving of a vestment.
Reply to Objection 5. The deacon's power is midway between the subdeacon's and the priest's. For the priest exercises a power directly on Christ's body, the subdeacon on the vessels only, and the deacon on Christ's body contained in a vessel. Hence it is not for him to touch Christ's body, but to carry the body on the paten, and to dispense the blood with the chalice. Consequently his power, as to the principal act, could not be expressed, either by the giving of the vessel only, or by the giving of the matter; and his power is expressed as to the secondary act alone, by his receiving the book of the Gospels, and this power is understood to contain the other; wherefore the character is impressed at the handing of the book.
Reply to Objection 6. The act of the acolyte whereby he serves with the cruet ranks before his act of carrying the torch; although he takes his name from the secondary act, because it is better known and more proper to him. Hence the acolyte receives the character when he is given the cruet, by virtue of the words uttered by the bishop.
Source: Summa Theologica (New Advent)