My good friend Dan Cruver had the privilege of writing the foreward and does an excellent job of summing up the importance of this subject for the church today:
Earthly adoption is horizontal. It is one human being establishing a relationship with another human being. Heavenly adoption is vertical. It is the eternal God graciously establishing a relationship with
fallen human beings, creatures who are by nature “children of disobedience” (Eph. 2:2) or “children of wrath” (Eph. 2:3).
God is an adoptive Father. Jesus, our Elder Brother, is God the Father’s eternal, only-begotten, natural Son. We believers are His children through adoption. This identity is central to who we are. As
adopted children, we enjoy all the rights and privileges of the relationship that God the Father enjoys with His eternal Son. This is an amazing reality and eternal privilege.
Adoption is heavenly before it is earthly. One is what God does; the other is what we do. Adoption is something God has done and is doing before it is something we have done and are doing. Adoption was invented by God even before He created the world.