TRỞ THÀNH NGƯỜI CÔNG GIÁO - BECOMING A CATHOLIC
The process by which adults come into the Church has come to be known as "the RCIA", which is short for "The Rite of Christian Initiation of  Adults."

Who is the process for?
1.  The unbaptized. The primary focus of the Rite of Christian  Initiation of Adults is on those who are not already Christian and have not been catechized.
2.  Baptized but uncatechized. Those who have been baptized either as Roman Catholics or as members of another Christian community but did not  receive further catechetical formation or instruction. These typically have also not celebrated confirmation nor Eucharist.
3.  Those seeking full Catholic Communion. These are baptized, practicing Christians from other denominations who seek entry into the Catholic  Church.
   In the case of children who have reached the age of reason, the proper pastor should be consulted for information about Baptism and the  other Sacraments of Initiation.
    The Rite of Christian Initiation of Adults is not for adult candidates for confirmation who have already received their First  Eucharist in the Catholic Church. They should have their own formation process and be confirmed at a celebration other than the Easter Vigil. 

What does the process look like?
The Rite of Christian Initiation is based on the principle that the  process of conversion proceeds gradually, in stages. Progress from one  stage to the next is marked by a liturgical celebration in the midst of  the parish community. The experience and needs of those in each category described above differ, and so the length of time may vary for each  person. Nevertheless, there are certain similarities among all the  groups and the process they will experience, and these can be listed as follows:

1. Precatechumenate

The first stage is called the period of inquiry (or the precatechumenate). This is when the individual first expresses an interest in becoming a Christian or a Catholic, and begins to explore,  with the help of the parish community, what his or her relationship with Christ might be and how that might be enriched and deepened by joining  this Christian community. There is no liturgical rite to mark the  beginning of this stage. This period of inquiry may last several months  or several years and ends either when the inquirer decides against   continuing in this direction or when the inquirer feels ready to move on and the community is prepared to welcome him or her.

2. Catechumenate

The second stage is called the catechumenate and, for the unbaptized listed above, who are now called catechumens, should last no less than one full year. For the baptized but uncatechized the period should be a similar length. For the candidates for full communion, this stage could  well be much shorter. The
Rite of Acceptance into the Order of Catechumens and the Rite of Welcoming mark the beginning of this stage. Catechesis for this period is rooted in the Lectionary and the Word as  it is proclaimed in the midst of the community. This is also a time for the catechumen or candidate to learn how to live as a Catholic Christian. This  period ends when the catechumens and candidates
express their desire to receive the sacraments of initiation and the community acknowledges their readiness.

3. Purification and enlightenment

The third stage is the period of purification and enlightenment and coincides with Lent. During this time the elect (formerly the  catechumens) and the candidates enter into a period of intense  preparation and prayer which includes the three public celebrations of  the scrutinies and is marked by the
presentations of the Creed and the  Lord's Prayer. The Rite of Election and the Call to Continuing  Conversion are celebrated at the beginning of this stage. This period  ends with the celebration of baptism, confirmation, and Eucharist at the Easter Vigil.
Note: only the elect are baptized. All the groups are confirmed and welcomed to the table.

4. Mystagogy

The fourth stage is the period of post baptismal catechesis or  mystagogy. At this time, the newly initiated explore their experience of being fully initiated through participation with all the faithful at  Sunday Eucharist and through appropriate catechesis. The period formally lasts through the Easter season and may be marked by a parish  celebration on or near Pentecost. On a more informal level, mystagogy is a lifelong process, one that all Christians are engaged in, as we all  work to deepen our sense of what it means to live the Christian life.

It is important to note that those who fall into the third category  above (candidates for full communion) do not always need to take part in the full  process. Especially if they have been actively living the Christian life in another denomination, they are likely in need of very  little catechesis
and may be welcomed into the Church on any Sunday  after a short period of preparation. According to the National Statutes for the chumenate,
"Those baptized persons who have lived as  Christians and need only instruction in the Catholic tradition and a  degree of probation within the
Catholic community should not be asked to undergo a full program parallel to the catechumenate." 
                                                                                                                                                                                     Source:  catholic.org