“In Him [Christ] you also, who have heard the word of truth, the gospel of your salvation, and have believed in Him, are now sealed with the promised Holy Spirit, which is the first installment of our inheritance toward redemption as God’s possession, to the praise of His glory.” (Ephesians 1:13-14)
The Sacrament of Confirmation seals the recipient with the gift of the Holy Spirit. This “sealing” brings into fullness the Sacrament of Baptism. Along with the Sacraments of Baptism and the Holy Eucharist, Confirmation, is one of the Sacraments of Christian Initiation.
Receiving the Sacrament of Confirmation is necessary for the completion of baptismal grace, and is administered by the laying on of hands by a Bishop in the Apostolic Succession.