Western History Begins with Him
Though there has been effort for some years to redefine the western calendar as divided between B.C.E. (before common era) and C.E. (common era) rather than B.C. (before Christ) and A.D. (Anno Domini, or year of our Lord), there can still be little doubt about the dividing line between the “before” and “after”; it is the approximate birth of Jesus, the God whose incarnation marks, for Christians, the watershed between lost and found, between law and grace, old and new, the world that was perishing and the world being renewed.
The Calendar Is Established
In western civilization, all history is anchored to this event. However, although the general sense is that A.D. 1 was Jesus' first birthday, in the Roman empire of the caesars the calendar was restarted every time a new emperor was installed. Dionysius Exiguus (translated as Dennis the Small) changed the Julian calendar's revision method, under direction of the Latin side of the church, in the sixth century. As this was long before the Gregorian calendar was introduced in 1582, but also long after Christianity was recognized as a religion tolerated by the Roman Empire in 313, Dionysius Exiguus' calendar had to be redacted back into historical records.
How accurate were Dionysius Exiguus' calculations?
Later scholarship found that the death of Herod the Great had taken place about five years earlier than Dionysius Exiguus' year 1, so his calendar is about five years off (since Herod was ruling when Jesus was born, according to Matthew's Gospel).
Dionysius Exiguus' calendar reform was the first time Jesus' estimated year of birth was officially fixed as the permanent beginning of the new era, though by that time many historians had already been referring to it as the great watershed of history.
Despite the biblical evidence confirming Jesus and his ministry, some still dispute whether there ever was such a person as Jesus of Nazareth who preached his own advent as the Jewish Messiah. And since the Enlightenment (1700s), there have been theologians who feel the Bible is unreliable, so their duty is to strip off the religious testimony about Jesus and research the historical evidence about him to determine the elusive truth.