The catechumenate is the process by which a candidate for baptism is formed and tested before being admitted to the Christian community, and for the first several centuries of the church it was long, structured and communal rather than a short private lesson.
The clearest early description comes from the Apostolic Tradition, a church order attributed to Hippolytus of Rome in the early third century, which sets out a catechumenate of up to three years combining moral instruction, examination of the candidates conduct by sponsors from the community, and an intensive final stage of prayer and fasting leading to baptism at Easter. Candidates were questioned and tested and could be turned away if their manner of life did not match what they claimed to believe. Admission was not automatic simply for asking.
The practice declined as the church itself changed. Once most of the population within Christian territories had been baptized as infants, the pool of adult candidates needing this kind of extended formation shrank to almost nothing, and by the early medieval period the catechumenate survived mainly as a set of ritual gestures folded into the infant baptism rite rather than as a lived process.
It was formally revived in the twentieth century following the Second Vatican Council, which restored a staged process of adult initiation built deliberately on the ancient model rather than inventing a new one. It is practiced today in the Roman Catholic Church and adapted, with variation, by several other Christian traditions that admit adult converts through a comparable period of instruction and discernment before baptism or reception.