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 4. Organization and Discipline of the Church” Loc. 13792-14769 (part 8).
§53. The Catholic Unity.
After a brief bibliography, Schaff describes the unity of the church based on Scripture, an episcopal government, and challenges which required rebuttal. By the time of the Nicene Creed, a solid confessional standard became the norm (Schaff 2014, Loc. 14377). The Church was rather uniformly seen as a cohesive body of people bound together by a common doctrine and practice. They would confess one, holy, catholic, and apostolic church (Schaff 2014, Loc. 14389). Schaff views this as a solid commitment in the ante-Nicene period. He cites the idea in Irenaeus particularly, but also in other Fathers (Schaff 2014, Loc. 14409).