In order to fully participate in the life of the Catholic Church, both children and adults must receive three sacraments of initiation. These sacraments act as a foundation in the spiritual life and are necessary to receive some of the other sacraments of the Church.
The Catechism of the Catholic Church briefly summarizes the importance of these sacraments.
The sacraments of Christian initiation — Baptism, Confirmation, and the Eucharist — lay the foundations of every Christian life. “The sharing in the divine nature given to men through the grace of Christ bears a certain likeness to the origin, development, and nourishing of natural life. The faithful are born anew by Baptism, strengthened by the sacrament of Confirmation, and receive in the Eucharist the food of eternal life. By means of these sacraments of Christian initiation, they thus receive in increasing measure the treasures of the divine life and advance toward the perfection of charity.”
Currently most children in the Roman rite receive these sacraments over a period of several years, while adult converts receive all three at the same time. This reflects an early Church tradition of initiating adults into the Catholic Church at the Easter Vigil, where they were baptized, confirmed and received the Eucharist.
This tradition of celebrating all three sacraments at the same time is also retained by Eastern Christians, who administer the sacraments of initiation to both infants and adults.
If someone wants to become a full member of the Catholic Church, these three sacraments complete Christian initiation and usher the individual into the heart of the Church.
How do I become Catholic?
How was baptism practiced in the early Church?