Schaff, Philip. History of the Christian Church (The Complete Eight Volumes in One). Amazon Kindle Edition, 2014.
Volume 2, Ante-Nicene Christianity A.D. 100-325, “Chapter 5. Christian Worship” (Includes an introduction and sections 59-74).
§67. Division of Divine Service. The Disciplina Arcani.
Schaff observes that by the mid-second century, Christian worship was divided into two portions. “The former consisted of scripture reading, preaching, prayer, and song, and was opened to the unbaptized and persons undergoing penance. The latter consisted of the holy communion, with its liturgical appendages: none but the proper members of the church could attend it; and before it began, all catechumens and unbelievers left the assembly at the order of the deacon, and the doors were closed or guarded” (Schaff 2014, Loc. 15214). Tertullian was particularly adamant that the unbaptized would not participate in the communion and its confession. The rite was treated very much as a hidden discipline. The term Disciplina Arcani was coined for this in 1679 (Schaff 2014, Loc. 1521). The practice essentially disappeared in the West after the sixth century. Schaff ties the disappearance to the decline of heathenism and universality of infant baptism.
Because the Sacraments were reserved for Christians, it was natural to show reverence and avoid offense by separating those who could not participate. Schaff does concede that the idea of a Secret Discipline has often been expanded to include all sorts of dogmas which may ot have been readily accepted by others (Schaff 2014, Loc. 15228). As Christianity spread in the West, the need to separate the baptized from catechumens became less important. Particularly as it became normal for almost everyone to be baptized as an infant rather than as a confessing convert, people of all ages were among the baptized (Schaff 2014, Loc. 15241). In the East a practice of dismissing catechumens has been retained.