The term refers to elements of certain Christian rituals and the teaching about them that are deliberately shrouded in mystery: this esp. regards baptism and the Eucharist and the formulas associated with them, such as the Our Father see Lord’s Prayer and the creed traditio and redditio Symboli. The discipline is considered an aspect of Christian catechesis, proposed for pedagogical reasons to safeguard the sublimity and venerable nature of the doctrine conveyed against a devaluation due to its being too widely known or its being confused with other, competing, pagan rituals. This explains the development of this discipline esp. during and not beyond the 4th c., contemporaneously with the growing spread of Christianity and the institutionalization and ritualization of the catechumenate. In fact, traces of a teaching for initiates, influenced by Judaism, are already present in certain sayings of Jesus e.g., Mt 13:11; Lk 8:10, of Paul e.g., 1 Cor 2:6ff. and 3:1-2, and esp. in the Apocalypse and apocrypha.
Reserve regarding certain notions concerning the Christian mysteries is also attested in the Apostolic Tradition 21, 3rd c.?. In the Alexandrian tradition and elsewhere an ascent to Christian knowledge by degrees is theorized. Clement of Alexandria, referring to 1 Cor 3, distinguishes common faith from perfect knowledge or “gnostic perfection”, not immediately accessible e.g., Strom. V 26,1.5. Origen also refers to Paul 1 Cor 2:6 to justify the transmission of the divine mysteries exclusively to those advanced in the faith, and he thus refutes Celsus, for whom an initiation only for the pure should be the case only in the pagan mystery cults, whereas sinners of every kind are called to Christian worship Cels. III 59-62.
The disciplina arcani was in fact influenced by the Hellenistic mystery cults, the principal medium through which the vocabulary and practice of secrecy in religious ritual reached Christianity, as a protection from profane external intrusions. Traces of esotericism were frequent in the Late Antique Greco-Roman religious world and were often seen as a means of attraction. Thus in the Christian setting we find the insistence on the duty to maintain secrecy concerning the ritual and meaning of baptism and the Eucharist, which seem to have been celebrated behind closed doors in the evening or at night Cyril Hier., Catech. 1,1; Ambr., Myst. 2, 5; Chrys., Hom. in Mt 23, 3. Basil of Caesarea emphasized both the value of the truths of the faith and that the rites be conveyed orally, and thus to be reserved in a halo of mystery and hiddenness e.g., the sign of the cross, the words of the epiclesis in the consecration of the eucharstic bread and chalice, the blessing of the baptismal water and oil, and various details of the administration of that sacrament; see Spir. 27,66. A mystagogy is needed for the penetration of these mysteries, like that in the baptismal catecheses of John Chrysostom e.g. Ad Illum. 1,4 and in those of JohnCyril of Jerusalem, who, e.g., says that he wanted to wait for the baptism of his interlocutors before revealing to them its deeper meaning Catech. 1,11; 2,1.