What is the meaning of Leaving the fold Why is called this?

"Leaving the fold" is an euphemism for leaving a church or religion. The terminology stems from the idea in Christianity that members of a church are sheep under their shepherd. This it taken from the biblical parable of "The Good Shepherd" where Jesus Christ, in telling the parable, represents himself as the Good Shepherd who lays down his life for his sheep, going out to look for the lost sheep who have left the fold (or the way of righteousness).

So to say that someone is leaving the fold, is to suggest that they have strayed from the paths of goodness, and have wandered fro the truth.