The early Church was without question a "missionary" Church, which not only welcomed, but actively sought new members. Many of the stories in the New Testament suggest that, the very first wave of conversion to Christianity was handled in a relatively simple matter. Basically, the converts would repent of their sins, be baptized and commit themselves to living their new life as Christians. It quickly became apparent however, that many of these new converts understood very little about the faith and oftentimes they even mixed various pagan practices and beliefs with their new religion. Many of these concerns were even addressed in scripture by St. Paul.
Another problem that the early Church faced was that of deadly persecution, which in turn created a climate of caution. Often they had to meet in secret, and had to be on guard against "spies" who would disguise themselves as new converts.
Aside from this caution, the thought of an unbelieving pagan consuming the body and blood of our Lord in the Eucharist was appalling to this first generation of Christians.
To help ensure that new converts truly understood their faith, that spies might actually be converted and to reduce the likelihood of a non-believer infiltrating the Eucharistic assembly the Church developed a much more careful process for conversion. The basic approach taken by the Church was to allow “catechumens”, the term used to designate those preparing to become Christians, to join in the first part of the liturgy - the Liturgy of the Word - but then to "dismiss" them at the beginning of the Liturgy of the Eucharist. The dismissed catechumens would be taken to a nearby room where they would reflect on the word and be taught the essentials of the Christian faith. After an extended period of preparation - usually three years - the catechumens would be fully initiated into their new faith at the Easter Vigil by celebrating the sacraments of baptism, confirmation and the First Communion.
This process of initiation fell out of use in later centuries, in no small part because of the reformation and the competition this presented for new members. However, this initiation process was restored by Vatican II and began reintroduction into the local parishes during the nineteen seventies and is our model for today’s RCIA.