St. Thomas Aquinas
“This sacrament has of itself the power of bestowing grace… when the sacrament itself is really received, grace is increased, and the spiritual life perfected: yet in different fashion from the sacrament of Confirmation, in which grace is increased and perfected for resisting the outward assaults of Christ’s enemies. But by this sacrament grace receives increase, and the spiritual life is perfected, so that man may stand perfect in himself by union with God.”
Source: St. Thomas Aquinas, Summa Theologica