In the Sacrament of Confirmation, Catholics receive the gift of the Holy Spirit to complete our incorporation into Christ’s body, the Church. We become true members of Christ’s body at our baptism, but Confirmation completes and confirms God’s grace of adoption, and our commitment to God.
By the sacrament of Confirmation, [the baptized] 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
Confirmation strengthens the life of grace and seals our commitment with the Holy Spirit.
But the one who gives us security with you in Christ and who anointed us is God; he has also put his seal upon us and given the Spirit in our hearts as a first installment.2 Corinthians 1: 21-22
This seal of the Holy Spirit marks our total belonging to Christ, our enrollment in his service for ever, as well as the promise of divine protection in the great eschatological trial”.CCC, 1296
As the gift of the Holy Spirit seals and completes our membership in the Church, confirmation enables and demands our sharing Christ with others. It was the gift of the Holy Spirit to the Apostles after Jesus ascended into heaven which got them to begin preaching the gospel to the world.
When the time for Pentecost was fulfilled, they were all in one place together. And suddenly there came from the sky a noise like a strong driving wind, and it filled the entire house in which they were. Then there appeared to them tongues as of fire, which parted and came to rest on each one of them. And they were all filled with the holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim.Acts 2:1-4
The symbolic action by which the Holy Spirit normally is given in Confirmation is a bishop laying his hands on the person he is confirming. This normally happens some time after a person is baptized as an infant in the Roman Catholic Church. This practice reflects that these two Sacraments were given separately in the early Church.
Now when the apostles in Jerusalem heard that Samaria had accepted the word of God, they sent them Peter and John, who went down and prayed for them, that they might receive the holy Spirit, for it had not yet fallen upon any of them; they had only been baptized in the name of the Lord Jesus. Then they laid hands on them and they received the holy Spirit.Acts 8:14-17
When adults become Roman Catholic, though, normally they receive this sacrament from a priest at the same time they are baptized, as do infants when they are baptized in the Orthodox and Eastern Catholic Churches.