As they say in the bible, that you're supposed to rejoice when people die and mourn when they're born, because it's one of the most painful acts you go through in life, is being born, and dying.
Although when Christianity appeared the total population of the planet was only a fraction of that of the twentieth century, most of the earth's surface was quite outside the Mediterranean world, Persia, India, and China.
This kind of forgetting does not erase memory, it lays the emotion surrounding the memory to rest.

