Catechesis (; from Greek: κατήχησις, "instruction by word of mouth", generally "instruction") is basic Christian religious education of children and adults, often from a catechism book. 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 Pope John Paul II's 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 ecclesiology, a catechumen (; via Latin catechumenus from Greek κατηχούμενος katēkhoumenos, "one being instructed", from κατά kata, "down" and ἦχος ēkhos, "sound") is a person receiving instruction from a catechist in the principles of the Christian religion with a view to baptism. The title and practice is most often used by Anglican, Lutheran, Methodist, Orthodox, Reformed/Presbyterian, and Roman Catholic Christians. Ecumenical organisations such as the North American Association for the Catechumenate are helping across several denominations to "shape ministries with adult seekers involving an extended time of faith formation and a meaningful experience of adult baptism at Easter."