In an interview with Ehrman by Ruth Graham (2014) with the Boston Globe, Ehrman stated:
The problem is that Jesus only makes claims for himself as being divine in the Gospel of John .... But what scholars have long noted is that Jesus doesn’t say any of those things in Matthew, Mark, and Luke and that Matthew, Mark, and Luke are [written] much earlier than John .... What I argue in the book (How Jesus Became God) is that it’s virtually inconceivable that if it was known Jesus called himself God, that Matthew, Mark, and Luke would just leave that part out.
The rationale behind this argument is that when John wrote his gospel much later the legends of Jesus’ resurrection and deity had formulated and infiltrated his gospel.
I will mention only two of many passages in the synoptic Gospels (Matthew, Mark, and Luke) where Jesus claimed deity and therefore, did not originate as a legend (Mark 2:1-12 and 14:62-63).
In Mark 2:1-12 (not the Gospel of John) Jesus stated that he forgave a paraplegic's sins. Immediately after Jesus forgave the paraplegic’s sins, the Pharisees responded, “Why does this man speak blasphemies? Who can forgive sins but God only?” They recognized that Jesus was acting like God who alone can forgive sins. They unknowingly spoke the truth about Jesus when they said only God can forgive sins. C. S. Lewis (1952) writes in Mere Christianity of the significance of Jesus telling a total stranger his sins were forgiven:
I mean the claim to forgive sins: any sins. Now unless the speaker is God, this is really so preposterous as to be comic. We can all understand how a man forgives offenses against himself. You tread on my toes and I forgive you, you steal my money and I forgive you. But what should we make of a man, himself unrobbed and untrodden on, who announced that he forgave you for treading on other men’s toes and stealing other men’s money? Asinine fatuity is the kindest description we should give of his conduct. Yet this is what Jesus did. He told people that their sins were forgiven, and never waited to consult all the other people whom their sins had undoubtedly injured. He unhesitatingly behaved as if He was the party chiefly concerned, the person chiefly offended in all offenses. This makes sense only if He really was the God whose laws are broken and whose love is wounded in every sin. In the mouth of any speaker who is not God, these words would imply what I can only regard as a silliness and conceit unrivaled by any other character in history. Yet (and this is the strange, significant thing) even His enemies, when they read the Gospels, do not usually get the impression of silliness and conceit. Still less do unprejudiced readers. Christ says that He is ‘humble and meek’ and we believe Him; not noticing that, if He were merely a man, humility and meekness are the very last characteristics we could attribute to some of His sayings (p. 51).
Ehrman's Boston Globe interviewer (2014) stated: I kept thinking of C.S. Lewis’s idea that Jesus was either a lunatic, a liar, or the Lord, and that there are no other possibilities.
Ehrman’s response: The problem is he left one option out. If you want to keep with “L” s, the option is “legend.” I’m not saying Jesus is a legend. But C.S. Lewis’s formulation is that Jesus called himself God, and if he called himself God, he was either telling the truth or he wasn’t. If he wasn’t telling the truth, he either knew it or he didn’t .... But Lewis doesn’t consider the idea that Jesus calling himself God could be legendary.
Again, in Mark, not the later Gospel of John, Jesus unequivocally claimed deity. The reason John recorded more claims, notice I said “more” not “all” claims, by Jesus to be God than the other Gospels is because that was John’s purpose in writing his Gospel as John 20:31 states.
Again in Mark 14:62-63 Jesus claimed to be God and was condemned to death for his blasphemy. The High Priest asked Jesus, "Are you the Christ the Son of the Blessed One?" And Jesus said, “I am.” John D. Grassmick in Bible Knowledge Commentary (1983) said, "The title Blessed One, found in this sense only here in the New Testament, is a Jewish substitute for God (cf. Mishnah Berachoth 7.3)" (p. 183). The answer Jesus gave to the question, "Are you the Son of God?" was "I am." Sounds like one of the "I am" claims of Christ in John's Gospel. The Pharisees and scribes recognized Jesus’ claim to deity and so should Bart Ehrman.
Here is C. S. Lewis's (1952) famous observation that Jesus' claim to be the Son of God made Him either a liar, or a lunatic, or Lord.
I am trying here to prevent anyone saying the really foolish thing that people often say about Him: “I’m ready to accept Jesus as a great moral teacher, but I don’t accept His claim to be God.” That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic—on a level with the man who says he is a poached egg—or else he would be the Devil of Hell. You must make your choice. Either this man was and is, the Son of God: or else a madman or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call Him Lord and God. But let us not come with any patronizing nonsense about His being a great human teacher. He has not left that open to us. He did not intend to (pp. 51-52).