Critics of Christianity are often very uncritical of their own belief system. I want to discuss an example that's relevant to the present Christmas season.
According to some critics of the Biblical accounts of Jesus' birth, Jesus most likely was born in Nazareth. How do they know? Because he's referred to as Jesus of Nazareth, the Nazarene, etc. I've discussed some problems with that view in previous posts, like here and here. Phrases like "Jesus of Nazareth" are far too ambiguous to carry the weight critics are suggesting, and Jesus is referred to that way by the same ancient sources who tell us that he was born in Bethlehem.
But let's overlook those problems for a moment. Think about how widespread the tradition of Jesus' Nazareth birthplace must have been under the critic's scenario. Supposedly, Jesus and his relatives, neighbors, etc. thought he was born in Nazareth for a few decades. The earliest Christians held the same view. Men like Peter and Paul held it as they traveled the world and established Christian churches. It was taught in the earliest gospel, Mark, which was highly influential in early Christianity according to the typical critic. The Nazareth birthplace was also taught in the last of the gospels, John, in the closing years of the first century. Think of how widespread the belief must have been, then. It was the only view or the dominant one for decades, and even at the end of the first century it was prominent enough to appear in a widely accepted gospel (John).
Yet, the tradition leaves no explicit trace in any New Testament document. Instead, critics have to argue that it's implied by references to "Jesus of Nazareth" and such. The Nazareth tradition is absent in the earliest extra-Biblical sources and beyond. The Bethlehem birthplace is explicitly and widely affirmed, in both West and East, by sources from a wide variety of locations, backgrounds, personalities, and theologies, including apocryphal documents, heretics, and people who didn't even profess to be Christians. The sources who discuss the Bethlehem birthplace seem to have no awareness of any significant dispute over the matter. See here for some examples and documentation.
If somebody is going to argue that belief in a Nazareth birthplace was widespread early on, he needs to explain why it left so little trace in the historical record. The early, explicit, and widespread affirmation of a Bethlehem birthplace makes sense if Jesus was born there. The evidence is what we'd expect under that scenario. It's not what we’d expect if he was instead born in Nazareth or somewhere else other than Bethlehem.
One way a skeptic could attempt to avoid the full force of the point I'm making is by conceding that there is no Nazareth tradition implied by "Jesus of Nazareth" and other such phrases. It could be argued that although Jesus wasn't born in Bethlehem, sources like the gospel of Mark and the gospel of John weren't claiming that he was born in Nazareth. Jesus was born somewhere other than Bethlehem, but the tradition of that other birthplace wasn't as widespread as the first critical scenario suggests.
That's a better argument. But it's still too problematic. For reasons I've discussed elsewhere, any early tradition of Jesus' birthplace is likely to have been widely known and widely discussed. Even if it wasn't widespread to the point of appearing in documents like the gospels of Mark and John, it still would have been widespread. So, if the dominant early tradition, or even a somewhat prominent one, was that Jesus was born somewhere other than Bethlehem, why does it leave so little trace in the historical record? And why are we supposed to be doubting the Bethlehem birthplace to begin with?