Why Jesus Christ was taken to divide the time to be AD and BC?

The terms AD and BC stand for "Anno Domini" and "before Christ". These were terms developed by the Catholic church when developing the Gregorian calendar in the sixth century.

"Anno Domini" does not mean "after death", but "in the year (anno) of our Lord (Domini)". So when Jesus was, say, 12 years old, that would have been 12 AD.

But modern scholarship has shown that the church made a mistake when calculating the year of Jesus' birth. It appears that he was actually born 4-6 years earlier than the calendar says.