One day in Korea, several leaders sat at dinner table with True Parents. Father asked one leader about his family. He said to Father that he and his wife are the first generation, his children are the second generation and his grand children the third generation. Father said you are wrong and said True Parents are the first generation in God's family. All those who received the Blessing, we become the second generation.
Blessed children become the third generation and our grand children become the fourth generation.
In order to be written in the book of God's Family, we need to be registered. In the bible it is mentioned about a Book of Life. Registration is entering the Book of Life. All those who are Blessed are children of God. Blessed families before the Registration Blessing ceremony are like illegitimate children of God.