Christianity fascinates me, particularly the question of how it came to believe what it did. For example I am eager to learn how modern theological beliefs compare to those of the early Christians. Unfortunately, we don't have any definitive answers, but historians try to ascertain the answers as best they can by tracing the bits and pieces of evidence that they find in the New Testament (at least for the earliest bits of evidence because there is nothing else form the early days).
Such a reconstruction is what Bart Ehrman has done in his latest book, How Jesus Became God. For Ehrman, it's not as simple as saying that He always was God. (It has been sometime since I finished the book, so I am going to give an overview as I remember it. I think I have got the gist of it, but I will likely muddle some particulars.)
Starting his analysis with the glimpses of interaction between Jesus and the disciples in the gospels, Ehrman posits that neither Jesus nor his disciples thought of Him as God. He was a Zealot, the Messiah whom God would use to usher in a new age of righteousness. He would act as God's agent in establishing the kingdom.
After the crucifixion the disciples believed they had seen the risen Christ. Ehrman does not dispute this. He thinks that the disciples, or at least some of them, came to believe this. Whether the appearances were real or imagined, in spirit or in body, the disciples thought that they had witnessed a risen Lord. Specifically, they soon came to believe that God had raised Jesus from the dead and given Him an exalted place in heaven. This is the adoptionist viewpoint that I have read (not in this book) that a few Christians hold even today. Jesus, therefore, became a divine figure of sorts but was not, at that time, held to be equal with God. Not at all.
Another minor step in the evolution of Jesus occurred soon afterward: that this adoption must have actually occurred when Jesus was baptized by John.
As time went on, and it seems to Ehrman that at least these first steps occurred quickly, Jesus began to be seen not just as one who had been lifted by God after death but as a being who must have always existed: perhaps like an angel who became incarnated as a man. The next step was for Him to have existed throughout all of time and to be God's agent of creation.
Finally, at the Council of Nicaea in 325 AD, it was agreed (by most) that Jesus must have been God himself, totally equal to God the Father: distinct but of the same substance.
What I say that it was decided, I don't mean that it was a sudden and rash decision for, obviously, these concepts had been gaining strength and didn't appear out of the blue. It wasn't brand new at Nicaea. It was also in this council that the trinity became an enshrined concept: the virgin birth too.We believe in one Lord, Jesus Christ,
the only Son of God,
eternally begotten of the Father,
God from God, Light from Light,
true God from true God,
begotten, not made ...
So, according to Ehrman, in 300 years Jesus went from being a putative human Messiah to the eternally preexistent Son of God who was Very God of Very God (to quote a common version of the Nicene Creed slightly different from the version that I posted above).
As I stated earlier, I think I have the gist of the chronology, but you would have to read the somewhat lengthy How God Became a Christian for yourself to truly follow the development. Interestingly, by the time Ehrman published his work, a group of conservative scholars had already posted a rebuttal book, How God Became Jesus. I haven't read it, but I have heard Ehrman discussiong/debating with one the authors. Curiously, he made his work available to them, even before it was officially published.