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.
§ 149. The Holy Trinity..
Having reviewed early Christian descriptions of the Father, Son, and Holy Spirit, Schaff observes that the formulation of the Trinity is complete, and that the teaching of the Trinity “has a peculiar, comprehensive, and definitive import in the Christian system, as a brief summary of all the truths and blessings of revealed religion” (Schaff 2014, Loc. 19571). However, the Scripture normally speaks obliquely about the Trinity, treating with one or another person of the Godhead at a time. Schaff sees the Trinity as that which clearly sets Christianity apart from all the other world religions (Schaff 2014, Loc. 19579).
Schaff notes some opinions that the Trinity came from Platonism, neo-Platonism, or from an Indian religion, but discounts those ideas out of thand (Schaff 2014, Loc. 19592). There may have been a predisposition to see plurality in divinity, but the Trinity is decisively a New Testament explication. Schaff further notes that theologians have frequently tried to find Trinitarian references in nature, though they regularly prove inadequate (Schaff 2014, Loc. 19612).
The Apostles’ and Nicene Creeds are arranged around the Trinity, which formulation also appears in many doxologies and other traditional statements of the Church (Schaff 2014, Loc. 19619). With all this evidence, the claims that the Trinity was a relatively late theological invention do not stand up to scrutiny. Schaff details the articulations of various Fathers, showing that they differ primarily in their means of explanation rather than in their underlying concepts.