Perhaps this isn’t adding too much to the conversation, but I’m pretty sure the Catholic Church was receiving folks long, long before there was a thing called RCIA. To be honest, I’ve always thought that RCIA was the “extraordinary” method; the one-on-one instruction and discernment with and by a priest seems much more in keeping with… I don’t know, little-t tradition?
Then again, I attended an FSSP parish for a long time. Maybe that’s just me, but RCIA seems to be vastly inferior to the depth and complexity of study that a person who is truly interested in the Faith can get on their own in a much shorter period of time.
When there are 20-30 or in some cases up to a hundred people wanting to be received into the Church at any given parish, and only one or two priests working in the parish, it isn’t feasible for each and every one of them to do one-on-one instruction with a priest - it’s a far better use of time to instruct them as a group, and to have a systematic process for bringing them into the life and practice of the Church.
Also, the one-on-one with a priest actually took
longer than the current model of RCIA - typically, one year to be baptized and receive First Holy Communion, and then a second year to prepare for Confirmation, which was done as a group at the Cathedral with the Bishop, at
his convenience (not at the demand of the convert), and then it was expected that the newly Confirmed adult would join a society for new converts, with weekly meetings for a third full year. There was nothing “quicker” about the one-on-one instruction with the priest - at least, not according to my reading. (
The Parish Priest’s Guide to Inquiry Classes, Paulist Press, 1960)
RCIA was the model used for the first five centuries of the Church, until most of Europe was Christianized (meaning, of course, Catholic). The one-on-one model was used from that time (to pick up the strays who’d missed the memo for whatever reason) until the early 1970s, when (possibly as a result of Vatican II) a lot more people became interested in becoming Catholic, and it was no longer a matter of one or two people about once every three years, but dozens of people, year after year.