Why set the birthday of Jesus as 0 years?

The reason why Jesus' birthday was set in AD 0 is that in Christian belief, Jesus is the savior of mankind, and his birth marks the beginning of human history. Christian doctrine holds that God created human beings and endowed them with life and free will. The birth of Jesus is the beginning when human beings are endowed with new life and free will.

In Christian doctrine, the birth of Jesus is regarded as an important historical event, which marks the beginning of human history. Christian doctrine holds that the birth of Jesus marks a new era for mankind, and mankind will usher in a new era. In this era, mankind will no longer be oppressed and discriminated in any form, but will gain freedom and dignity.

Therefore, it is to commemorate this important historical event to set Jesus' birthday as the year 0. At the same time, it is also a belief in Christian doctrine that the birth of Jesus is an important milestone in human history and will lead mankind to a new era.