Confirmation completes the grace of Baptism as we are enriched by a special outpouring of the gifts of the Holy Spirit through the laying on of hands and anointing by the bishop, which seals and confirms the baptized in union with Christ as they are sent forth to bear witness to Christ by word and example.
As St. Ambrose says, "Recall then that you have received the spiritual seal, the spirit of wisdom and understanding, the spirit of right judgment and courage, the spirit of knowledge and reverence, the spirit of holy fear in God’s presence. Guard what you have received. God the Father has marked you with his sign; Christ the Lord has confirmed you and has placed his pledge, the Spirit, in your hearts."