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 12. The Development of Catholic Theology in Conflict with Heresy” Sections 137-158, Loc. 18758-20235.
§ 154. Other Doctrines.
Schaff notes that many doctrines which were apparently believed among early Christians were not clearly codified and explained in the Fathers, “including faith, justification, and sanctification” (Schaff 2014, Loc. 19866). The more subjectively understood doctrines were among the last to be clearly articulated. The more objective, the earlier the doctrine is explained.