Luke 2:3

Every one into his own city. Among the Romans it would probably have been sufficient for every man to register in the city where he might be resident at the time, rather than in the city of his ancestral home. It is known that the usual Roman “enrollment” by cities was not always followed in the provinces. For example, the Gauls were “enrolled” by tribes. An extant decree authorizing a Roman census in Egypt required people to register at their place of origin (see bibliography entry for Caird, p. 265). In view of the fact that tribal genealogy meant so much to the Jews, it may well have been that Herod the Great decided on “enrollment” by tribes as the best procedure for his realm. At any rate, the mention of this point is indirect testimony pointing to Herod as the one through whom the Roman decree was executed in Judea, and also vindicating the reliability of Luke’s account.