The BC/AD division was invented by a Scythian monk named Dionysius Exiguus in 525 CE — roughly five centuries after the events it marks. He was working on calculating the dates of Easter and wanted to replace the existing system that counted years from the reign of the Roman emperor Diocletian, who had persecuted Christians. He preferred to count from the Incarnation of Christ instead.
His dating was also probably wrong by a few years — most modern scholars place Jesus’s birth somewhere between 6 and 4 BCE based on the Herod reference in the Gospels, meaning the calendar Dionysius created was already slightly off from the start.
The system spread gradually through the medieval period, largely through Bede’s use of it in his Ecclesiastical History of the English People in 731 CE. From there it became standard across Christian Europe over the following centuries.
The calendar shift reflects Jesus as a hinge point in history
Even secular historians acknowledge that Western civilization’s entire subsequent development is incomprehensible without the Christ event at its center.
The calendar reflects a cultural recognition, even if the mechanism was a monk doing Easter calculations rather than a conscious decision by humanity.
Something fundamental changed in human history, a new era that is “Anno Domini” - In the Year of our Lord