This was the first registration when Quirinius was governor of Syria.
Luke 2:2

Biblical detractors point to this verse as evidence of error, because Quirinius wasn’t governor of Syria until much later and took his census in 6 AD, not 4 BC. However, they are making the absurd mistake of assuming that they already know everything about history that there is to know. It turns out that Quirinius was governor of Syria twice with a break in between.