The reception of the sacrament of Confirmation is necessary for the completion of baptismal grace.
Baptism, the Eucharist, and the sacrament of Confirmation together constitute the “sacraments of Christian initiation,” whose unity must be safeguarded. For “by the sacrament of Confirmation, [the baptized] are more perfectly bound to the Church and are enriched with a special strength of the Holy Spirit. Hence they are, as true witnesses of Christ, more strictly obliged to spread and defend the faith by word and deed.”
The effect of the sacrament of Confirmation is the special outpouring of the Holy Spirit as once granted to the apostles on the day of Pentecost. Like Baptism which it completes, Confirmation is given only once, for it too imprints on the soul an indelible spiritual mark, the “character,” which is the sign that Jesus Christ has marked a Christian with the seal of his Spirit by clothing him with power from on high so that he may be his witness. This “character” perfects the common priesthood of the faithful, received in Baptism, and “the confirmed person receives the power to profess faith in Christ publicly and as it were officially.
Every baptized person not yet confirmed can and should receive the sacrament of Confirmation. Without Confirmation and Eucharist, Baptism is certainly valid and efficacious, but Christian initiation remains incomplete.
A candidate for Confirmation who has attained the age of reason must profess the faith, be in the state of grace, have the intention of receiving the sacrament, and be prepared to assume the role of disciple and witness to Christ, both within the ecclesial community and in temporal affairs.
Adapted from the Catechism of the Catholic Church