Baptism, the Eucharist and the sacrament of Confirmation together constitute ‘the sacraments of Christian initiation,’ whose unity must be safeguarded. It must be explained to the faithful that the reception of the sacrament of Confirmation is necessary for the completion of baptismal grace. For by the sacrament of Confirmation, the baptised are more perfectly bound to the Church and are enriched with a special strength of the Holy Spirit. Hence they are, as true witnesses of Christ, more strictly obliged to spread and defend the faith by word and deed.

– Catechism of the Catholic Church 1285

Catechism of the Catholic Church

1315 “Now when the apostles at Jerusalem heard that Samaria had received the word of God, they sent to them Peter and John, who came down and prayed for them that they might receive the Holy Spirit; for it had not yet fallen on any of them, but they had only been baptized in the name of the Lord Jesus. Then they laid their hands on them and they received the Holy Spirit” (Acts 8:14-17).

1316 Confirmation perfects Baptismal grace; it is the sacrament which gives the Holy Spirit in order to root us more deeply in the divine filiation, incorporate us more firmly into Christ, strengthen our bond with the Church, associate us more closely with her mission, and help us bear witness to the Christian faith in words accompanied by deeds.

1317 Confirmation, like Baptism, imprints a spiritual mark or indelible character on the Christian’s soul; for this reason one can receive this sacrament only once in one’s life.

1318 In the East this sacrament is administered immediately after Baptism and is followed by participation in the Eucharist; this tradition highlights the unity of the three sacraments of Christian initiation. In the Latin Church this sacrament is administered when the age of reason has been reached, and its celebration is ordinarily reserved to the bishop, thus signifying that this sacrament strengthens the ecclesial bond.

1319 A candidate for Confirmation who has attained the age of reason must profess the faith, be in the state of grace, have the intention of receiving the sacrament, and be prepared to assume the role of disciple and witness to Christ, both within the ecclesial community and in temporal affairs.

1320 The essential rite of Confirmation is anointing the forehead of the baptized with sacred chrism (in the East other sense-organs as well), together with the laying on of the minister’s hand and the words: “Accipe signaculum doni Spiritus Sancti” (Be sealed with the Gift of the Holy Spirit.) in the Roman rite, or: Signaculum doni Spiritus Sancti [the seal of the gift of the Holy Spirit] in the Byzantine rite.

1321 When Confirmation is celebrated separately from Baptism, its connection with Baptism is expressed, among other ways, by the renewal of baptismal promises. The celebration of Confirmation during the Eucharist helps underline the unity of the sacraments of Christian initiation.

When the apostles in Jerusalem heard that Samaria had accepted the word of God, they sent Peter and John to Samaria. When they arrived, they prayed for the new believers there that they might receive the Holy Spirit, because the Holy Spirit had not yet come on any of them; they had simply been baptized in the name of the Lord Jesus. Then Peter and John placed their hands on them, and they received the Holy Spirit.

– Acts 8:14-17

The grace conferred by the Sacrament of Confirmation is more specifically a gift of strength. This gift corresponds to the need for greater zeal in facing the ‘spiritual battle’ of faith and charity in order to resist temptation and give the witness of Christian word and deed to the world with courage, fervor and perseverance. This zeal is conferred by the Holy Spirit.

– Saint Pope John Paul II (L’osservatore Romano, 8 Apr 92)

The perfection of spiritual strength consists properly on a man’s daring to confess the faith of Christ in the presence of anyone at all, and in a man’s being not withdrawn therefrom either by confusion or by terror, for strength drives out inordinate terror. Therefore, the sacrament by which spiritual strength is conferred on the one born again makes him in some sense a front-line fighter for the faith of Christ. And because fighters under a prince carry his insignia, they who receive the Sacrament of Confirmation are signed with the Sign of the Cross by which He fought and conquered. This sign they receive on the forehead as a sign that without a blush they publicly confess the faith of Christ.

– Saint Thomas Aquinas (Summa Contra Gentiles)

Forget not the Holy Spirit at the moment of your [Confirmation]; He is ready to mark your soul with His seal…. He will give you the heavenly and divine seal which makes the devil tremble; He will arm you for the fight; He will give you strength.

– Saint Cyril of Jerusalem (Seventeenth catechesis on the Holy Spirit)