Adoption
Adoption is giving to someone the name, and place and privileges of a son who is not a son or daughter by birth.
Mankind is adopted into the family of God by the process of redemption through the Holy Spirit.  God legally accepts a repenting sinner and makes him a member of His universal family, acknowledging him as a son and or daughter born in the family, and giving him all the rights and privileges of a son and daughter.  An adopted child takes the same name as the rest of the family, shares in the same inheritance, privileges, and daily benefits provided for all, and becomes subject to the responsibilities and paternal (received or inherited from one's male parent) care of the one who adopted him.  The adoption is a words used only by Paul and is used of the believer when the question of rights and privileges is concerned (Rom 8:15, 23; 9:4; Gal 4:5; Eph 1:5)
"Adoption" is a term involving the dignity  (being worthy, honored, or esteemed) of the relationship of believers as sons, it is not a coming into the family by spiritual birth, but being put into the position of sons.
Digging deeper:

Paul understood the Roman culture and thus is why he used the word “adoption”.  When a child was born into a Roman family, especially a son.  He would one day have everything that his father had.  But up until then he had tutors (teachers), people to show him how to do certain things.  When he reached the year of accountability  and that was determine by his father way be for he was even born.  Then he was adopted into his family and had the rights of being a son. 

A good example of this was in the 1959 movie, “Ben- Hur”.  When this slave became a son through adoption, he had all the same rights as the father’s real son.

The scriptures tell us that through the Law, it was a tutor, a teacher to bring us to being adopted and then become a son.  Not a child of the King, but a son of the King.  So at the time of being “born again” into God’s Kingdom we become His son or daughter and have the same rights as His first begotten Son, Jesus.  Kewl

Adoption is the act of God whereby He legaly accepts a repenting sinner and makes him a member of His universal family, acknowledging him as a son being born in the family, and giving him all the rights and privileges of a son.  The adopted child takes the same name as the rest of the family, shares in the same inheritance, privileges, and daily benefits provided for all, and is subject to the responsibilities and paternal care of the one who adopted him.  We are adopted into the family of God by the process of redemption by the Holy Spirit, by our faith.  Adoption is simply the placing of one in a family whereby he will becomes a son in the natureal, but with God he is already a son, not becomes a son.  It is a position, one does not grow into it.

The word "adoption" is a word used only by Paul and is used of the believer when the question of rights and privileges is concerened.

Rom. 8:15, 23;  9:4;   Gal 4:5; and Eph 1:5