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.
§ 148. The Holy Spirit.
Schaff notes that controversy about the Holy Spirit was largely absent prior to the 4th century, which can explain why there were relatively few attempts at explanation (Schaff 2014, Loc. 19518). He does not take this in any way as an indication that there was less of a Trinitarian understanding of theology, merely that there was no need to defend the nature of the Holy Spirit. The ante-Nicene understanding was “that the Holy Spirit, the sole agent in the application of redemption, is a supernatural divine being, and taht he is an independent person; thus closely allied to the Father and the Son yet hypostatically different from them both” (Schaff 2014, Loc. 19531). However, in some instances the Holy Spirit was seen as subordinate to the Father and the Son. Schaff cites Justin Martyr as holding and explaining this view (Schaff 2014, Loc. 19538). Clement of Alexandria and Origen also tend to this weakness, while still confessing the deity and eternity of the Holy Spirit. Irenaeus comes closer to a Nicene understanding (Schaff 2014, Loc. 19565). Again, the careful explanation seems to emerge from questions and disputes.