Trinity: Why did Christians develop such a different understanding of God?

All of Jesus' original apostles were Jewish. Of the documents in the New Testament, only two (Luke and Acts) were known to be written by a Gentile. To the best of our knowledge, the rest of the New Testament was written by people who were life-long Jews, who continued to see themselves as Jews while understanding Jesus as the Messiah. In the Second Temple era there were different Jewish groups with different opinions on various points of law and texts, but none of those mainstream groups disputed monotheism: There is only one God, and God is one. 

From that background, why not keep to the most obvious solution: That Jesus is simply human, and so the Jewish understanding of God remains unchanged? It's important to any following discussion that we take the first step seriously and make the first point clear: Why is there a need for any discussion at all? The whole question could have been a non-starter; possibly the most natural view of that topic would have been as a non-starter; so why did something else happen? 

Or to come from the other direction, we can look at the alternatives to the Trinity that have been considered. Over the centuries, Christians wrestled with other options such as: Maybe Jesus was fully divine and his humanity was merely an appearance; or maybe Jesus was adopted by God; or maybe Jesus was the one through whom all things were made as the first of God's creations. Still in our times we see alternatives like: Maybe Jesus was an angel, or maybe Jesus is a god but not the God of this world, or maybe God is one and Jesus is one way that God appears to us. (These are my fumbling attempts to summarize the views of Jehovah's Witnesses, Mormons, and Oneness Pentecostals; if anyone can suggest a clearer way to word those, or finds a more accurate way to summarize, those suggestions would be welcome.) 

Why take a quick survey of the alternatives? To show that even the Christians who have not agreed with the doctrine of the Trinity generally don't say that Jesus is simply human. Even among feuding sects who hesitate to recognize each other as belonging to the same religion, there's an unintended consensus on that one point: There's more going on with Jesus than "simply human". 

If we were to make a decision tree of how people understand him, the first point might be, "Is Jesus simply human?" At that point, not only the prevalent Christian theologies but most of the alternative Christian theologies join together in the same answer: No. 

For this blog, most of the readers are familiar with the reasons why the answer is generally: No. In the New Testament, the early records of Jesus introduce him with John the Baptist fulfilling prophecies about preparing the way for the LORD -- texts that in the original language use the Divine Name. Even in the shortest, possibly least-theological gospel in the New Testament, that of Mark, we find his opponents challenging him over whether he is laying claim to God's authority: "Who can forgive sins but God alone?" And even in that early document we find words attributed to Jesus in which he questions their understanding of the promised Messiah: "Why does David call him Lord?" 

The alternative gospels -- the ones outside the New Testament -- do not present us with a merely human Jesus either. Consider the Coptic Gospel of Thomas: "Jesus said, 'I am the light that is over all things. I am all: from me all came forth, and to me all attained. Split a piece of wood; I am there. Lift up the stone, and you will find me there.'" (Saying #77)

These few texts are not presented in order to prove any point, but as a sample of the reasons why someone might look at the documents and think: Jesus' followers didn't think he was simply human; and unless they were inventing what he said then Jesus didn't consider himself simply human either. That lies at the heart of why Christians developed a different understanding of God, and to some extent still wrestle with it today: How exactly do we understand the information that we have, being faithful to the facts as we know them? 

(Part of a series)