The Shaw Pocket Bible Handbook
, Walter A. Elwell, Editor, (Harold Shaw Pub., Wheaton , IL; 1984), p. 346
The process through which a person who does not belong to a given family is formally brought into it and made a full, legal family member with the rights and responsibilities of that position. The practice of adoption was not common among the Jews, but was more widespread in the Greek and Roman world. The apostle Paul used the term to illustrate the truth that believers have been given the status of "sonship" in the heavenly family; they can call God "Father" (Rom. 3:15>; Gal. 4:6>). Adoption makes it clear that our sonship is conferred on us, in distinction from Christ's, which is inherent.