The Apostolic Tradition of Hippolytus Translated into...

In the apostolic age converts were accepted with little question and were baptized immediately on profession of faith;[199] the missionary zeal of the new religion, 86 heightened by the expectation of the end of the world, sought only to compel men to come in. Naturally this enthusiasm was always tempered with common sense—no teacher could have baptized every applicant—but the doors were opened wide, and the New Testament gives no hint of any formal training before reception. The hope that defects would be made up by Christian grace was doubtless fulfilled to a surprising degree, but it was also often grievously disappointed: men were admitted into Christianity who neither understood its teachings nor desired to follow them, and it was from this class that Gnosticism and other vagaries drew their recruits. The account in Acts 8. 18-24 is typical.

The result was a violent reaction that made entry into the church extremely difficult, and no one was permitted baptism until he had passed through a long and searching probation called the “catechumenate”. As it appears fully developed in the early third century, it must reach far back into the second or perhaps even into the first.