The earliest known example of a Catechetical work is the “Teaching of the Twelve Apostles,” which Athanasius names among the “books not included in the Canon, but appointed by the Fathers to be read by those who are just recently coming to us, and wish to be instructed in the word of godliness (κατηχεῖσθαι τὸν τῆς εὐσεβείας λόγον).” This use of the Didache for the instruction of recent converts from Paganism agrees with its original purpose as stated in the longer title, “Teaching of the Lord through the Twelve Apostles for the Gentiles.” The first six chapters are evidently adapted for those who need elementary instruction, more particularly for Catechumens of Gentile descent, as distinct from Jewish candidates for Baptisms. The remaining chapters of the Didaché relate chiefly to the administration of Baptism, to Prayer, Fasting, and the services of the Lord’s Day, and to the celebration of the Agape and Eucharist. This same division of subjects is observed in the two classes of S. Cyril’s Catechetical Lectures: the first class, including the Procatechesis, consists of XIX Lectures addressed to candidates for Baptism, and these are followed by five “Mystagogic” Lectures, so called as being explanations of the Sacramental Mysteries to the newly-baptized.
— Edward Hamilton Gifford, “The Catechetical Lectures of S. Cyril: Introduction,” in S. Cyril of Jerusalem, S. Gregory Nazianzen, ed. Philip Schaff and Henry Wace, vol. 7, A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series (New York: Christian Literature Company, 1894), xii.