There must have once been a conference to settle how the Calendar would be carved up between the religions.
Envoys would have been sent by the Classical pagans, the Nordic pagans, and the ambitious Christian religion. I picture these skilled negotiators posing for the photographers, like at Yalta.
The Nordic envoy, the most dominant, would have demanded the names of the days. The daily cycle would mean the most to people, and if each day was named in honour of the Nordic religion, then it would survive for evermore. The Nordic envoy got what he wanted.
The Classical envoy, the most beautiful, asked politely for the names of the months. The monthly cycle, because of the moon and other reasons too delicate to mention, would mean the most to people. Each month will be named after a Classical great, and so the Classical religion would survive for evermore. The Classical envoy got what she wanted.
The Christian envoy, the most intelligent, was relieved and delighted. Without a further word, she took the years.
It was a wise choice. Her BC/AD system provides a bias towards Christianity, at least in the West.
Few people care for the names of the days of the week or the months of the year, which may as well be named after the seven dwarfs, or the twelve men in a cricket team (including the twelfth man), for all the practical difference it makes to how we now think about religion.
The reckoning of years is different.
The wise Christian envoy passed her BC/AD system to Dionysius Exiguus in around 525 AD, who popularised it.
In a subtle way, this BC/AD system validates the Christian church.
The early church is about 100-400 AD, which is of course just right for an early church, and the life and times of Jesus and his followers will be earlier still, which is perfect.
Religions with their origin later, or during the counter-intuitive reverse-dating BC period before, seem wrong because they are clearly too late or too early.
The effect would perhaps be different if we had carried on with the classical Olympiad system, which commenced in 776 BC (and ended in 392 AD).
Here Jesus of Nazareth and his followers would be around in the eighth to ninth century, and the "early church" would be from around 900 to 1200. They would then seem just another bunch of latter-day religious fanatics.
However, it is sadly not sensible to make any significant change to the AD dating, at least after the sixth century AD. (However, do spare me the politically correct CE ("common era") pandering.)
But do we really need to continue with BC and early AD dating, at least in histories of the Classical world?
Why can't histories of the Classical period use the calendars which (usually) meant something to those alive at the time? Historians could switch to the Christian dating when Dionysius Exiguus, Bede, and others come along. There could be a converison table, just as there is for other measurements.
I wonder if the Christians would ever let go of their BC/AD dating system?
After all, it does give the Christians a historical head start.