The following are excerpts from the Catechism of Trent (also called the "Roman Catechism").
Importance Of Instruction On Confirmation
If ever there was a time demanding the diligence of pastors in explaining the Sacrament of Confirmation, in these days certainly it requires special attention, when there are found in the holy Church of God many by whom this Sacrament is altogether omitted; while very few seek to obtain from it the fruit of divine grace which they should derive from its participation.
Lest, therefore, this divine blessing may seem, through their fault, and to their most serious injury, to have been conferred on them in vain, the faithful are to be instructed both on Whitsunday, on which day it is principally administered, and also on such other days as pastors shall deem convenient. Their instructions should so treat the nature, power, and dignity of this Sacrament, that the faithful may understand not only that it is not to be neglected, hut that it is to be received with the greatest piety and devotion.
Name of this Sacrament
To begin with the name, it should be taught that this Sacrament is called by the Church Confirmation because, if there is no obstacle to the efficacy of the Sacrament, a baptised person, when anointed with the sacred chrism by the Bishop, with the accompanying solemn words: I sign thee with the sign of the cross, and confirm thee with the chrism of salvation, in the name of the Father, and of the Son, and of the Holy Ghost, becomes stronger with the strength of a new power, and thus begins to be a perfect soldier of Christ.
Sponsors at Confirmation
A sponsor is also required, as we have already shown to be the case in Baptism. For if they who enter the fencing lists have need for some one whose skill and counsel may teach them the thrusts and passes by which to overcome their adversaries, while remaining safe themselves; how much more will the faithful require a leader and monitor, when, sheathed, as it were, in the stoutest armour by this Sacrament of Confirmation, they engage in the spiritual conflict, in which eternal salvation is the proposed reward. With good reason, therefore, are sponsors employed in the administration of this Sacrament also; and the same spiritual affinity is contracted in Confirmation, which, as we have already shown, is contracted by sponsors in Baptism, so as to impede the lawful marriage of the parties.
Dispositions For Receiving Confirmation
From this, therefore, it follows that persons of mature age, who are to be confirmed, must, if they desire to obtain the grace and gifts of this Sacrament, not only bring with them faith and piety, but also grieve from their hearts for the serious sins which they have committed.
The pastor should take care that they have previous recourse to confession of their sins; should exhort them to fasting and other works of piety; and admonish them of the propriety of reviving that laudable practice of the ancient Church, of receiving this Sacrament fasting. It is to be presumed that to this the faithful may be easily persuaded, if they but understand the gifts and admirable effects of this Sacrament.
The Effects of Confirmation
Pastors, therefore, should teach that, in common with the other Sacraments, Confirmation, unless some obstacle be present on the part of the receiver, imparts new grace. For we have shown that these sacred and mystical signs are of such a character as to indicate and produce grace.
The Grace Of Strength
But besides these things, which are common to this and the other (Sacraments), it is peculiar to Confirmation first to perfect the grace of Baptism. For those who have been made Christians by Baptism, still have in some sort the tenderness and softness, as it were, of new-born infants, and afterwards become, by means of the Sacrament of chrism, stronger to resist all the assaults of the world, the flesh and the devil, while their minds are fully confirmed in faith to confess and glorify the name of our Lord Jesus Christ. Hence; also, originated the very name (Confirmation), as no one will doubt. For the word Confirmation is not derived, as some not less ignorantly than impiously have pretended, from the circumstance that persons baptised in infancy, when arrived at mature years, were of old brought to the Bishop, in order to confirm their faith in Christ, which they had embraced ill Baptism, so that Confirmation would seem not to differ from catechetical instruction. Of such a practice no reliable testimony can be adduced. On the contrary, the name has been derived from the fact that by virtue of this Sacrament God confirms in us the work He commenced in Baptism, leading us to the perfection of solid Christian virtue.
Increase In Grace
But not only does it confirm, it also increases (divine grace), as says Melchiades: The Holy Ghost, whose salutary descent upon the waters of Baptism, imparts in the font fullness to the accomplishment of innocence, in Confirmation gives an increase of grace; and not only an increase, but an increase after a wonderful manner. This the Scriptures beautifully express by a metaphor taken from clothing: Stay you in the city, said our Lord and Saviour, speaking of this Sacrament, until you be clothed with power from on high.
If pastors wish to show the divine efficacy of this Sacrament — and this, no doubt, will have great influence in affecting the minds of the faithful — it will be sufficient if they explain what occurred to the Apostles themselves. So weak and timid were they before, and even at the very time of the Passion, that no sooner was our Lord apprehended, than they instantly fled; and Peter, who had been designated the rock and foundation of the Church, and who had displayed unshaken constancy and exalted magnanimity, terrified at the voice of one weak woman, denied, not once nor twice only, but a third time, that he was a disciple of Jesus Christ; and after the Resurrection they all remained shut up at home for fear of the Jews. But, on the day of Pentecost, so great was the power of the Holy Ghost with which they were all filled that, while they boldly and freely disseminated the Gospel confided to them, not only through Judea, but throughout the world, they thought no greater happiness could await them than that of being accounted worthy to suffer contumely, chains, torments and crucifixion, for the name of Christ.
Character Of Soldier Of Christ
Confirmation has also the effect of impressing a character. Hence, as we before said of Baptism, and as will be more fully explained in its proper place with regard to the Sacrament of Orders also, it can on no account ever be repeated.
If, then, these things be frequently and accurately explained by pastors, it will be almost impossible that the faithful, having known the utility and dignity of this Sacrament, should not use every exertion to receive it with purity and devotion.