Catechesis (/ˌkætəˈkiːsɪs/; from Greek: κατήχησις, "instruction by word of mouth", generally "instruction") is basic Christian religious education of children and adults. It started as education of converts to Christianity, but as the religion became institutionalized, catechesis was used for education of members who had been baptized as infants. As defined in the Catechism of the Catholic Church, paragraph 5 (quoting John Paul II, Apostolic Exhortation Catechesi tradendae 18):
Catechesis is an education in the faith of children, young people and adults which includes especially the teaching of Christian doctrine imparted, generally speaking, in an organic and systematic way, with a view to initiating the hearers into the fullness of Christian life.
In the Catholic Church, catechist is a term used of anyone engaged in religious formation and education, from the bishop to lay ecclesial ministers and clergy to volunteers at the local level. The primary catechists for children are their parents or communities. Protestant churches typically have Sunday School classes for educating children in religion, as well as adult classes for continuing education.
In the early church, new converts, referred to as "catechumens," were instructed (catechized) in the basic elements of the faith such as the Apostles' Creed, Lord's Prayer, and sacraments in preparation for baptism. During the fourth and fifth centuries, baptism had become a several-week-long rite leading up to the baptism on Easter. During this time, catechumens attended several meetings of intensive catechetical preaching, often by the bishop himself, and often accompanied by special prayers, exorcisms, and other rites. Catechumens recited the Apostles' Creed on Holy Saturday to show that one had completed catechetical instruction. By the sixth century, most of those presented for baptism were infants, and pre-baptismal catechesis was abandoned. The decline of preaching and education in general following the barbarian invasions also affected the decline of catechesis. Later, instructors (catechists) would teach Christians who had been baptized as children, to prepare for practicing the religion as thinking persons, both older children and adults. The word "catechism," used for a manual for this instruction, appeared in the Late Middle Ages. During this time the instruction was also expanded to include memorization of the Lord's Prayer and Apostles' Creed. Some clergy probably provided expositions of this material in addition to the Ten Commandments. The main function of catechesis during this period was preparation for confession by enabling the Christian to identify their sins.