Catechesis (/ˌkætəˈkiːsɪs/; 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 (/ˌkætɪˈkjuːmən, -mɛn/; 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."

The word catechumen comes from the passive form of the Greek word κατηχέω (katēcheō), which is used seven times in the New Testament. In the passive, it means "to be instructed, informed."

The catechumenate slowly developed from the development of doctrine and the need to test converts against the dangers of falling away. The Bible records (Acts 19) that the Apostle Paul, while visiting some people who were described as "disciples", established they had received the baptism of John for the repentance of sins but had not yet heard of or received the Holy Spirit. Further, from the second century it appears that baptisms were held only at certain times of year, indicating that periods of instruction were the rule rather than the exception. The Catholic Encyclopedia notes: "As the acceptance of Christianity involved belief in a body of doctrine and the observance of the Divine law ("teach, make disciples, scholars of them"; "teaching them to observe all things whatever I have commanded you", Matthew 28:20 [see Great Commission]), it is clear that some sort of preliminary instruction must have been given to the converts." See also Council of Jerusalem. Justin Martyr, in his First Apology, cites instruction as occurring prior to baptism:

The "persuasion" would be carried out by the preaching of an evangelist; but since belief must precede baptism, the person concerned should be prepared spiritually to receive the indwelling of the Holy Spirit through baptism. That person would receive the sign of the Cross and possibly aspersion with holy water from a minister, indicating their entry to the state of catechumen.

In the early church, 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. Catechumens were limited as to their attendance in formal services. As unbaptized, they could not actively take part in any service, for that was reserved for those baptized. One practice permitted them to remain in the first part of the mass, but even in the earliest centuries dismissed them before the Eucharist. Others had them entering through a side door, or observing from the side, from a gallery, or near the font; while it was not unknown to bar them from all services until baptized.