During his lifetime, Jesus didn't call himself God and didn't consider himself God...and none of his disciples had any inkling at all that he was God.
I found this statement to be rather remarkable, for reasons I will explain later. However, I think it would be helpful first to understand the context of Professor Ehrman's assertion. Based on my reading of the NPR webpage, Professor Ehrman appears to be focusing his argument on the so-called Synoptic Gospels (Matthew, Mark, and Luke). These three gospels are called synoptic (meaning something like "similar view") due to the fact that they contain a great deal of identical or similar material about the life of Jesus. Professor Ehrman's contention is that in none of the Synoptic Gospels is Jesus quoted as calling himself God. He concedes that in the Gospel of John Jesus is quoted as claiming to be God (e.g., "Before Abraham was, I am," "I and the Father are one," "If you've seen me, you've seen the Father"), but since the Gospel of John was written some time after the Synoptic Gospels, its historical accuracy is suspect. Professor Ehrman goes on to argue that it seems highly unlikely that if Jesus had called Himself God the writers of the Synoptic Gospels would have not taken note of this fact. Thus, it is reasonable to conclude that Jesus never claimed to be God. He goes on to speculate that Christians began claiming that Jesus was God in response to the official dogma of the Roman Empire that the emperor was a god.
Now, I have to say that I find Professor Ehrman's overall argument highly problematic. For one thing, it does not automatically follow that the Gospel of John is less reliable than the Synoptic Gospels just because it was written after them. My understanding is that most biblical scholars date the Gospel of John to (at the latest) the AD 90s (we know it couldn't have been written later than this due to the fact that the oldest copy still in existence of a part of the New Testament, the so-called John Rylands manuscript, consists of several verses from the Gospel of John and has been conclusively dated to the early 100s). The Synoptic Gospels have been dated anywhere from the early 60s to the 80s. Consequently, there is only a difference of a few decades between the Gospel of John and the Synoptic Gospels. How likely is it that the author of John could have gotten away with painting a radically different portrait of Jesus (a divine Savior rather than a human teacher) only a few decades after the composition of the Synoptic Gospels? Moreover, the author of the Gospel of John claims to be an eyewitness to many of the things Jesus said and did. Another problem with the professor's argument is that when the Romans spoke of the emperor as a god, they did not mean the same thing that Christians meant when they called Jesus God. The Romans' belief in the supposed divinity of the emperor was rooted in polytheism. On the other hand, the Christians' belief in the divinity of Jesus was rooted in Jewish monotheism (remember that most of the original Christians were Jews). In other words, for the Romans, the emperor was one god out of many gods, while for Christians, Jesus was the one and only true God--not the same thing!
Let us return now to the quotation from Professor Ehrman that appears at the beginning of this post. In this quotation, he asserts three things: (1) Jesus never called Himself God, (2) Jesus never considered Himself to be God, and (3) Jesus's disciples didn't think He was God (during His lifetime). Since Professor Ehrman essentially dismisses the Gospel of John as having little historical validity, his argument appears to be based on what is recorded about Jesus in the Synoptic Gospels. First of all, regarding Ehrman's third claim--that Jesus's disciples didn't believe He was God during His lifetime--I actually think he has a valid point. Throughout the Gospels (including the Synoptic Gospels), Jesus's disciples, prior to the Resurrection, appear not to view Him unmistakably as God. They do seem to view Him as the Messiah (the Christ), the special one sent from God, but my understanding is that in popular Jewish belief of the time, it was assumed that the Messiah would be human, not divine. However, this fact doesn't prove much, except that the disciples, with their limited human understanding, couldn't easily grasp who Jesus truly was. It took the post-Resurrection appearances to make clear to the disciples that Jesus was, in fact, God.
Second, regarding Professor Ehrman's first claim--that Jesus (as portrayed in the Synoptic Gospels) never called Himself God--I would say that on a certain level this is true. To the best of my knowledge, nowhere in the Synoptic Gospels does Jesus say directly He is God. The problem for Professor Ehrman's argument is that even in the Synoptic Gospels, Jesus often acts and speaks as if he were God. For example, in the Synoptics we find Jesus claiming the authority to forgive sins (Mark 2:5-6; Luke 5:20-21) and saying that in the future He will judge all mankind Matthew (25:31-46). For the Jews, these were the sole prerogatives of God Himself. In the famous Sermon on the Mount (Matthew 5-7), Jesus places His words on par with those words believed by the Jews to have been spoken to Moses by God Himself, recorded in the Torah (Old Testament). For example, He tells his listeners: "You have heard it was said to the people long ago, 'You shall not murder [Exodus 20:13], and anyone who murders will be subject to judgment. ' But I tell you that anyone who is angry with a brother or sister will be subject to judgment" (Matthew 5:21-22).
Perhaps even more to the point is the reaction to Jesus's words during His trial before the Jewish Sanhedrin (high council). In both the Gospels of Matthew (chapter 26) and Mark (chapter 14), the Jewish high priest puts Jesus under oath to tell him whether Jesus claims to be the Son of God. Although the words Jesus is quoted as using in response differ somewhat in the two Gospels, their essential meaning is the same--Jesus does claim to be the Son of God. Moreover, He goes on to say "you will see the Son of Man [Jesus's favorite expression for Himself] seated at the right hand of Power [i.e., God] and coming with the clouds of heaven." The response of the high priest is to tear his clothes (a sign of great distress and/or anger) and to call Jesus' s words "blasphemy." This is important because from the perspective of the Jews at that time blasphemy did not only mean cursing God or misusing His name, but also any act that offended against the majesty of God. It is difficult not to conclude that the reason why the high priest considered Jesus' s words blasphemous is that he understood Jesus to be claiming to be no less than God Himself. So, the case can be made that even in the Synoptic Gospels Jesus is depicted as claiming (by implication) to be God, even if He doesn't directly say He is God.
Finally, regarding Professor Ehrman's second claim--that Jesus never considered Himself to be God--I have to say that my first reaction when reading this assertion was to ask myself how Ehrman could claim to know Jesus's thoughts. Was he claiming an ability to read minds? More seriously, the fact is that the only way we can know what Jesus thought about Himself is through the words of His that are recorded in the Bible. As I believe I have shown in the previous paragraph, Jesus certainly did speak (and act) as if He viewed Himself as God, which would seem to contradict Ehrman's claim. The only way to deny this would be to argue that the writers of the Synoptic Gospels fabricated the actions and sayings of Jesus that imply that He viewed Himself as divine. However, to do so would undermine Ehrman's argument since he is basing his argument on the assumption that the Synoptic Gospels present a fairly accurate account of what Jesus said. Actually, it seems to me that Professor Ehrman's views regarding the historical reliability of the Synoptic Gospels is inconsistent. On the one hand, for the sake of his argument that Jesus never claimed to be divine, he seems to accept the reliability of the Synoptic Gospels (as opposed to the Gospel of John) in reporting Jesus's words. On the other hand, at another point in the interview he casts doubt on the idea that Jesus's body was buried in a tomb after the Crucifixion, despite the fact that such a burial is described in the Synoptics. Consequently, it would appear that he believes that the Synoptic Gospels are not historically reliable. Now, I suppose he could say that some parts of the Synoptic Gospels are historically accurate and other parts are not, but then I think he would owe us a principled explanation as to why this would be the case. Otherwise, it would seem that he was cherry-picking only those passages that appeared to support his thesis.
In short, Professor Ehrman's argument is less convincing than it might seem at first glance. Indeed, I believe I have demonstrated that the question asked at the top of the NPR webpage--"If Jesus didn't call [emphasis mine] himself God, how did he become one?"--reflects some highly debatable assumptions. Moreover, I should point out that the fact that Jesus's followers initially struggled to understand how He could be God doesn't mean that He wasn't God--it is human experience that sometimes a truth takes time to be grasped. Furthermore, given the context of Jewish monotheism out of which Christianity developed, it seems hardly likely that the earliest Christians would have just invented the idea of Jesus's divinity--it appears more likely that there must have been something (the Resurrection) that forced them to that conclusion. I only wish NPR had given an opportunity for an orthodox Christian scholar (e.g., William Lane Craig) to respond to Professor Ehrman or even to engage in a debate with him--I think that would have been much fairer and more enlightening to NPR's listeners.
Image: The Four Evangelists by Jordaens Louvre (1593-1678) from commons.wikimedia.org