How do you join the Catholic Church?
Rite of Christian Initiation of Adults
The Rite of Christian Initiation of Adults (RCIA), or Ordo Initiationis Christianae Adultorum (OICA) is a process developed by the Catholic Church for prospective converts to Catholicism who are above the age of infant baptism. Candidates are gradually introduced to aspects of Catholic beliefs and practices. The basic process applies to adults and older children, with younger children initiated through an adapted version sometimes incorrectly referred to as the Rite of Christian Initiation of Children (RCIC).
Ecumenism
One problem with the larger numbers is that single denominations can be counted multiple times. For example, the Catholic Church is a single church, or communion, comprising 24 distinct self-governing particular churches in full communion with the bishop of Rome (the largest being the Latin Church, commonly called "Roman Catholic"). Further, the Catholic Church presence in each country is counted as a different denomination—though this is in no way an ecclesiologically accurate definition. This can result in the one Catholic Church being counted as 242 distinct denominations, as in the "World Christian Encyclopedia".