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).
§72. Catechetical Instruction and Confirmation.
From the apostolic period onward, training people before their baptism was considered important. Schaff notes that in the Book of Acts, we see, among others, Theophilus and Apollos receiving instruction. Catechesis would precede adult baptism. Schaff does say, “at a later period, after the general introduction of infant baptism, it followed” (Schaff 2014, Loc. 15481). “The catechumens, or hearers, were regarded not as unbelievers, but as half-Christians, and were accordingly allowed to attend all the exercises of worship, except the celebration of the sacraments” (Schaff 2014, Loc. 15488). Regardless of social or academic background, those who came for baptism would receive training. Schaff finds a typical pattern of two years’ instruction (Schaff 2014, Loc. 15496).
Schaff finds confirmation as the “positive complement” to baptism. It normally accompanied baptism but was later separated (Schaff 2014, Loc. 15496). After the third century, confirmation became the power of the bishop. This is the case in the Anglican communion still, but other groups have extended it to priests and deacons (Schaff 2014, Loc. 15504).