Fahrenheit or centigrade? Inches or centimeters? All these scales have to use some arbitrary increment as their reference. It doesn't really matter what, so long as we can all agree on what it is.
The centigrade scale is based on the melting point of ice/water being defined as 0C and the boiling point of water being defined as 100C. One gram of water occupies one cubic centimeter. This standard makes it easy for any lab, to build a thermometer, a measuring cup or a meter stick from scratch, if need be.
In terms of AD and BC, Rome was the pinnacle of ancient civilization. It incorporated elements from many of the previous peaks of civilization like Greek, Egyptian, Persian. Rome took this to the next level and controlled and influenced the western world.
The rise of Christianity had a profound impact on Rome. It led to political divisions due to Christianity being known as the religion of the slaves. The political turmoil led to the decline of Rome. Rome which was never replaced by another culture from the ancient world.
Eventually Rome incorporates Christianity in the 4th century AD. This led to the transitional time of the dark ages, from which the modern era appears. AD and BC is not an arbitrary point in time, but like centigrade, it was a way to calibrate the evolution of humanity.
The atheist religion is shallow, They feel if you call a goat, a bird, and the goat can fly. They fail to see that there is substance that goes beyond the facade of things.
