Confirmation
"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 grace of our Baptism, which makes us children of God. Just as the Holy Spirit descended on the disciples at Pentecost, so He is given to us in Confirmation to make us witnesses of all that God has done. The Old Testament prophesied that the Spirit of God would descend on the Messiah and would be poured out on the messianic people as a gift.