"The word katechesis means instruction by word of mouth, especially by questioning and answering. Though it may apply to any subject-matter, it is commonly used for instruction in the elements of religion, especially preparation for initiation into Christianity. The word and others of the same origin occur in St. Luke's Gospel: "That thou mayest know the verity of those things in which thou hast been instructed" (katechethes, in quibus eruditus es — i, 4). In Acts 18:25, Apollo is described as "instructed [katechemenos, edoctus] in the way of the Lord". St. Paul uses the word twice: "I had rather speak five words with my understanding, that I may instruct [katecheso, instruam] others also" (1 Corinthians 14:19); and "Let him that is instructed [ho katechoumenos, is qui catechizatur] in the word, communicate to him that instructeth [to katechounti, ei qui catechizat] him, in all good things" (Galatians 6:6). Hence the word, with its technical meaning of oral religious instruction, passed into ecclesiastical use, and is applied both to the act of instructing and the subject-matter of the instruction."

(Thomas Scannell, "Christian Doctrine," The Catholic Encyclopedia. Vol. 5.)