Bart Ehrman recently wrote a Christmas article for Newsweek. He repeats common objections to the infancy narratives without interacting with the counterarguments. From reading Ehrman, you wouldn't know that even many non-conservative scholars argue for conservative conclusions about the narratives, such as Jesus' Davidic ancestry and the Bethlehem birthplace. Ehrman's article doesn't break any new ground, and its coverage of the old ground doesn't go much beyond repeating the old objections in an introductory manner. For those who are interested in responses to such objections, as well as replies to more advanced ones, see our archive of Christmas material here.
I'll respond further to a couple of Ehrman's comments, starting with the following:
"This [Luke's census account] is not a story based on historical fact. It is a narrative designed to show how Jesus could have been born in Bethlehem—whence the Messiah was to come—when everyone knew in fact that he came from Nazareth." (page 3)
Luke wouldn't need a census in order to place Jesus in Bethlehem, as Matthew's infancy narrative illustrates. If Luke's census account is as historically problematic as Ehrman claims it is, then Luke probably wouldn't have chosen such an event to make up in order to get Jesus to Bethlehem. There would have been many better alternatives available. And Ehrman fails to interact with the many ancient sources who affirm the historicity of the census and fails to address the lack of ancient non-Christian opposition to the account. See my post on the census here and the larger series on the subject linked within that post.
If Ehrman is saying that "everyone knew in fact that he [Jesus] came from Nazareth" in the sense that it was known that he was there in his childhood, then consider the implications of Ehrman's claim. If such an aspect of Jesus' childhood was a known fact that Luke and other early Christians couldn't or wouldn't dispute, then why are we supposed to believe that they were much more willing and able to distort other facts about Jesus' childhood? If a childhood in Nazareth was widely remembered and unalterable, why think that other memories of Jesus' childhood were as malleable as Ehrman and other critics suggest?
On the other hand, if Ehrman is just saying that Jesus was known to have come from Nazareth later in life, then so what? Why would Luke have to make up an account of a census in Jesus' infancy in order to get around something known about Jesus' location much later in life?
Matthew and Luke both have Jesus in Nazareth early in life, in spite of a birth in Bethlehem. That's an unusual scenario. It's doubtful that they both would have hit upon that scenario independently in the process of making up stories in an atmosphere in which there was little concern for historicity. Why not just have Jesus in Bethlehem for his whole life or for most of his life, for example?
Ehrman goes on to say:
"The accounts of Jesus’ life in the New Testament have never been called 'histories'; instead, they have always been known as ¬'Gospels'—that is 'proclamations of the good news.' These are books that meant to declare religious truths, not historical facts." (3)
The fact that a document conveys good news doesn't suggest that it isn't meant to convey history as well. There's widespread agreement among modern scholars, including non-conservatives, that the gospels are Greco-Roman biographies. That's a historical genre. See here.
Furthermore, the earliest post-apostolic sources repeatedly use terms other than "gospels" to refer to the documents. For example, consider Justin Martyr's use of the term "memoirs":
"Justin counters this by implying that the Gospel accounts are historically reliable in the ordinary way of such accounts. The gospels were written by Jesus' disciples or their successors, who faithfully and reliably remembered what Jesus had said and done. There is nothing more to it, and nothing more is needed. Justin evidently sees considerable argumentative value in the fact that these Memoirs [the gospels] were put into writing at an early stage, by Jesus' closest disciples, the apostles, or by their immediate followers. We therefore do not have to rely on oral tradition only, transmitted through a large number of intermediary transmitters." (Oskar Skarsaune, in Sara Parvis and Paul Foster, edd., Justin Martyr And His Worlds [Minneapolis, Minnesota: Fortress Press, 2007], 73)
Justin also refers to the documents as gospels, not just memoirs, but the two aren't mutually exclusive. For example, Origen, while addressing a matter conveyed in the infancy narratives, refers to "the history recorded in the Gospels" (Against Celsus, 1:51). Many other comments of a similar nature were made by other early Christians.
The early enemies of Christianity responded to the gospels as if they were making historical claims. Think of Justin's interaction with Trypho and Origen's interaction with Celsus.
Ancient Jewish Messianic expectations involved a historical figure and historical prophecy fulfillment. The Old Testament scriptures that the New Testament documents were in some ways modeled after were perceived as historically reliable accounts of God's activity in history.