Read - Reading: Lost Christianities: Christian Scriptures and the Battles Over Authentication by Bart D. Ehrman (The Great Courses)

Lecture 24: Early Christian Creeds
The final lecture considers the formation of the Christian creeds: statements of faith to determine what was true (orthodox) and what was false (heretical). The well-known creeds of the 4th century, such as the Nicene Creed, developed from earlier formulations known as the "Rule of Faith," and from confessions by converts before baptism.

All that work to create coherent creeds in the 2nd-4th century and yet we again have a crazy diversity centuries later.