That is a question that Christians answer in a very different way than someone like Dr. Ehrman. I am reading and reviewing the book by Dr. Bart Ehrman, Jesus Interrupted.
I don’t always read a book straight through because in many books there is much paper giving little information that I want to read. I move to what I hoped would be an interesting chapter: Who Invented Christianity? Ehrman starts the chapter with an interesting proposition to Christians: Why don’t they read their book? That is a great question—we deserve to have to answer that question seriously and often.
However, overall, this a very lackluster chapter. There is nothing new, different, or interesting in this chapter—old arguments many writers have delivered before. It is a sleeper of a chapter.
“When did Jesus become the Son of God?” is a question that Ehrman asks but I am not sure why he even wants to bring the subject up because we know he does not believe that Jesus is the Son of God. Christians who know the Bible know that Jesus did not become the Son of God. Dr Ehrman disappoints in this chapter because he showed his lack of understand of the texts, of God, and of Christianity in general.
Dr. Ehrman does assume here that the historical Jewish view (though we know there were variations in Jewish views) was that the Messiah was to be a conquering king. We can deduce that the reason that they had this view is because they listened to teachers who had this view and who did not integrate key passages of the Jewish Bible into a theology of the Messiah. This would explain why many Jews totally missed Jesus as their Messiah. For all the other reasons, read the New Testament.