Admission into the Catholic Church can take one of several paths:
The most frequent path is through infant baptism, when parents bring their infant child for Baptism.
If an individual is not baptized as an infant, either in the Catholic Church or in any of the mainstream Trinitarian Christian communities (Lutheran, Episcopalian, Presbyterian, etc.), he/she is invited to journey through the process of the Rite of Christian Initiation of Adults (R.C.I.A.), through which they seek to be welcomed into the Church through the three Initiation Sacraments of Baptism, Confirmation, and First Eucharist.
If an individual is baptized in one of the mainstream Trinitarian Christian communities, but wishes to be accepted into the Catholic Church, alongside the R.C.I.A. is the process of coming to full communion in the Church. Because the Church recognizes his/her Baptism (we proclaim "one baptism for the forgiveness of sins"), an invitation is extended to a process to full communion through the Sacraments of Confirmation and Eucharist, in addition to being introduced and invited to participate in the Sacrament of Reconciliation.