The main effect of the Sacrament of Confirmation is the special outpouring of the Holy Spirit that was also given to the Apostles on the day of Pentecost.
This outpouring of the Holy Spirit clothes us in the armor of God, giving us a “special strength of the Holy Spirit to spread and defend the faith by word and action as true witnesses of Christ, to confess the name of Christ boldly, and never to be ashamed of the Cross” (CCC 1303).
The outpouring of the Holy Spirit received in Confirmation also increases in us the Gifts of the Holy Spirit, first received at Baptism.