In the Acts, where they talk about the conversion and subsequent baptism, the people were already practising Jews, like all the Apostles. They already knew their Scripture and the Law. All they had to do was accept Jesus as the Messiah who had been prophesied for centuries.
In the early church, when they were dealing with converts from paganism, the people asking for baptism knew nothing about the faith and therefore required a long catechesis in order to learn about the faith. In the early church, people sometimes had to spend years in study before receiving the sacraments of Baptism and Holy Communion. Indeed, they were not even allowed to be present at the Consecration.
Nowadays, people are converting from other Christian denominations as well as athiests or agnostics or even converting from non-Christian faiths like Islam, or Buddhism or Wicca. Some need to ‘un-learn’ things that have been taught in error, some need to learn it all in the first place. They all need to learn what the Church truly teaches. This takes time.
It would be doing them a great disservice to deny them the full knowledge of the faith they desire to join.
