Rite of Christian Initiation of Adults, RCIA, is for Baptized Christians wishing to become Catholic.
"The rite of Christian initiation presented here is designed for adults who, after hearing the mystery of Christ proclaimed, consciously and freely seek the living God and enter the way of faith and conversion as the Holy Spirit opens their hearts. By God's help they will be strengthened spiritually during their preparation and at the proper time will receive the sacraments fruitfully." -RCIA, #1
RCIA is also the process by which validly baptized Christians can explore becoming Catholic - and then become Catholic if they choose to do so.
The Rite by which they become Catholic is not baptism, but is called “Reception into the Full Communion of the Catholic Church.”
There is no hurry to become Catholic, no obligation or pressure, and neither is there unnecessary delay for these who are ONE with us in baptism.
“Reception of Baptized Christians into the Full Communion of the Catholic Church is the liturgical rite by which a person born and baptized in a separated ecclesial Community is received, according to the Latin rite, into the full communion of the Catholic Church. The rite is so arranged that no greater burden than necessary (see Acts 15:28) is required for the establishment of communion and unity.” -RCIA, #473
Christians of other denominations fall into one of two different categories:
1. Catechized: Those who are baptized, worship actively, and who have participated actively in formal “formation” in faith are considered “catechized” Christians who already have a relationship with Jesus Christ and the Church. These Christians are to be received into Full Communion as quickly as possible – as quickly as they can fully consent to the totality of Catholic belief and Tradition, calling to mind often the grace of their baptism and shaping their lives according to Catholic practice. When they are thus disposed, they make their first sacramental Confession. Soon afterward, usually at a Mass, they will make a Profession of Faith as a Catholic, be received into the Full Communion of the Catholic Church, be Confirmed, and receive Communion as a Catholic for the first time.
2. Uncatechized: Those who are baptized but who have had little experience of worship, little involvement in their Church and less than six years of formal “formation” in the faith are considered “graced” by baptism, but “uncatechized”. These baptized persons require a full year in the RCIA. When the time for full communion is at hand, those who are baptized will make a first Confession in the week or so leading to their Reception into full communion of the Catholic Church. Then, all in one Mass, they will make a Profession of Faith as a Catholic, be received into Full Communion, be Confirmed, and finally celebrate Eucharist within the Mass for the first time.After receiving the sacraments, the neophytes continue in RCIA for another 50 days, as they reflect on what has happened through the sacraments.