By Confirmation, Christians, that is, those who are anointed, share more completely in the mission of Jesus Christ and the fullness of the Holy Spirit with which he is filled, so that their lives may give off “the aroma of Christ.” By this anointing the confirmand receives the “mark,” the sealof the Holy Spirit. A seal is a symbol of a person, a sign of personal authority, or ownership of an object. Hence soldiers were marked with their leader’s seal and slaves with their master’s. A seal authenticates a juridical act or document and occasionally makes it secret. Christ himself declared that he was marked with his Father’s seal. Christians are also marked with a seal: “It is God who establishes us with you in Christ and has commissioned us; he has put his seal on us and given us his Spirit in our hearts as a guarantee.” This seal of the Holy Spirit marks our total belonging to Christ, our enrolment in his service for ever, as well as the promise of divine protection in the great eschatological trial.
(Catechism of the Catholic Church, paragraphs 1294-1296)