Question: "Why does the Orthodox Church require adults who are preparing for baptism to become catechumens, and to wait before they are allowed to be baptized. This requirement does not fit the pattern of conversions in Acts. On the day of Pentecost, about the 3000 that were baptized the same day Peter preached to them (Acts 2:37-42). The Samaritans who Philip preached to were baptized when they believed (Acts 8:12). The Ethiopian eunuch was baptized the same day (Acts 8:26-39). Cornelius, a gentile, was baptized the same day after he met Peter (Acts 10:44-48). The Philippian jailer who was baptized within a few hours of believing (Acts 16:31-34). Why was there a change in practice?"
Most of the people mentioned above were people who had a a prior knowledge of Judaism. For a Jew, or a Gentile God-fearer, becoming a Christian was not entering into completely foreign territory. The only exception is the Philippian jailer, whose conversion was a rather dramatic one. However, when you have people with pagan backgrounds who became interested in Christianity, but who had no background to immediately understand it, it makes sense that the Church would normally want there to be a period of time in which such people would be instructed in the Faith prior to being admitted to the sacraments. Consider the fact that St. Paul taught that if someone partakes of the Eucharist unworthily, eats and drinks damnation to themselves (1 Corinthians 11:27-29) -- wouldn't the Church want to give a convert from paganism some time in order to come to understand what the Eucharist means, before putting them into a position in which they likely might unworthily partake of it?
As time went on the experience of the Church also confirmed the need for converts from paganism to be well instructed before being baptized, because when persecutions arose, so many of them fell away, because they were not sufficiently grounded in the Faith.
Christ gave the Apostles the power to bind and to loose (Matthew 16:19; 18:18; John 20:23), and the Church teaches that this power was passed down through their successors, the bishops. And so even during the time of the Apostles, the Church has made decision and established standards that "seemed good to the Holy Spirit, and to us" (Acts 15:28). In fact, the same Church that decided which books belonged in the Bible, also decided that the catechumenate was a good idea when receiving adults converts. The length of time someone should remain a catechumen depends on their background, and also on how diligent they are in studying the Faith.
