I don't think Christianity evolved in a fluid fashion at all. I think the beginnings of Christianity came from the mystery schools of the Serapis cult and then gnostic ideas came in along with the thoughts of people like Plato. When Constantine made the religion his choice his friend Eusebius probably convinced him of this idea and then he had to make up some doctrine to established the church and religion. When this was done people from all over jumped on the band wagon and brought their beliefs and cults with them.
Christianity was influenced by cults like that of Dyonisus and Mithra and the the holy Church of Rome which worshiped the Cesar's. Most biblical writings before the 6th century were just scraps of paper and even the image of Christ before then was that of a lamb on a cross. The oldest bibles even after the 6th century don't have the name of Jesus or anything close to that in Greek near as much in the scriptures as we are led to believe. Just a four letter title . The church also had to keep having councils to make changes to what was to be taught. If the gospels were written in the later part of the first century like we've been told these questions and concerns as to what to teach could easily been answered just by reading the gospels.
Christianity's evolution was very rocky and very slow going. Before 325 CE the numbers of Christians must have numbered in just the thousands, in my opinion.