I'm not sure what Luke was thinking, but it's mostly our own reading of 2:1 that assumes Luke meant "the whole world, all at the same time". Yes, the next line flows into "everyone" going home, but the third line jumps ahead to Quirinius in AD 6, whereas Augustus' new census was probably put into effect during the 20's BC. Whatever Luke thought he was accomplishing, that's a long span of history being collapsed into not many words. If the first and third line are 30 years apart, how can we decide whether that "everyone" belongs at one end and not the other?
If the conservative grammar police would allow Luke the soft fallacy of awkward usage, everything would be simple. If Luke tried to say (or meant to say) that the Jesus-birth-census was "before" that more famous one two decades later - that's "before" and not "first" or "when" - then these awkward book ends would clearly reveal themselves, merely, as broad scale historical framing. Luke would then only be saying, my story begins after Augustus changed censusing but before Quirinius put down Judas' rebellion.
That is, it could all be so simple if we allow that Luke apparently slipped into coherent but nonstandard grammar. Alas, theologians apparently require an inerrant linguistics much more than a coherent accounting of actual events. Tis pity, tis true. Some of them feel that way about *you*, too. (Caring much more that you say the right words than what you do with your life.)
Nevertheless, despite Luke's lack of clarity he does reference the Jesus-birth-census 4 times in 5 verses, so let's home in on that. If Augustus decided to count Herod's subjects, it had to be punishment for the alleged offense of 9/8 BC, the precise years when the Proconsul of Syria was Gaius Sentius Saturninus, Augustus' former brother-in-law, the one whom Tertullian cites as being actually responsible for the Jesus-birth-census.
Besides, that fruit basket turnover plan was so ludicrous, it must have been unique (at least, to that point). Personally, I suspect Saturninus came up with it on his own. He was probably trying to be thorough, since there were no previous records to go by, but the logistical and scheduling nightmare that surely ensued does make him look like a bit of an idiot. As it happens, Saturninus isn't known for any notable accomplishments in his long career babysitting important provinces. Ah, nepotism!
To sum up: I don't care so much about explaining what Quirinius is doing in verse three. What matters much more is that - as it so happens - we do in fact possess a coherent accounting of factual data to explain how and why Joseph had to go down to Bethlehem. That Mary came too probably means the census was their excuse to relocate and leave scandal-town.
Faith doesn't and shouldn't depend on having facts to support what we might as well already believe... but such background evidence is still very nice when we happen to have it. Don't you think?
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