Smith's Bible Dictionary
Adoption. An expression used by St. Paul in reference to the present and prospective privileges of Christians. Romans 8:15; Romans 8:23; Galatians 4:5; Ephesians 1:5. He probably alludes to the Roman custom by which a person not having children of his own might adopt as his son one born of other parents. The relationship was to all intents and purposes the same as existed between a natural father and son. The term is used figuratively to show the close relationship to God of the Christian. Galatians 4:4-5; Romans 8:14-17. He is received into God's family from the world, and becomes a child and heir of God.
These files are public domain.
Smith, William, Dr. Entry for 'Adoption'. Smith's Bible Dictionary. https://www.studylight.org/dictionaries/eng/sbd/a/adoption.html. 1901.