Young Virgins Lotila Why Did Moses Keep All The Young Virgins Alive, But Kill Everyone Else?

Why did Moses keep all the young virgins alive, but kill everyone else? - young virgins lotila

"Now all kill male among the children, kill, and every woman, man lay with him is known. But all the children of women who take even a man lying down with him recognized that life for themselves."
Numbers 31:17-18