Marcus Peter Johnson has some helpful comments on adoption in his book One with Christ which I recently reviewed.
‘The term translated “adoption” in the New Testament is unique to Paul’s letters. The Greek term is huiothesia, which Paul uses five times (Rom. 8:15; 9:4; Gal. 4:5; Eph. 1:5). It is a compound of huios (“son”) and thesis (“placing”), and could be literally rendered “placed as sons,” although there is some disagreement among biblical scholars about how best to translate it.’ (157)
‘There may indeed be some merit in attempting to understand Paul’s use of huiothesia against the cultural-linguistic backdrop of his day, but there is compelling reason to think that his use of the term was influenced far more my theological considerations than cultural ones. In other words. when Paul speaks of Christians as “placed as son,” he has at the forefront of his mind our being place in the Son, Jesus Christ.’ (157)
‘As such, Paul is not “reaching” for cultural analogies as conceptual bridges to explain what it means that we are adopted by God; rather, he is working with a more basic theological notion: the Father-son relationship that is intrinsic to God’s own being, and which we come to share by incorporation into Christ.’ (157)
The fact that God adopts us and we thereby become children of God is based in the reality of God’s own relationship with his Son. It is not a reality that is derived from God external to himself – a category of blessing that God creates outside of the Father-Son relationship internal to his being – but an existence that is derived from within God’s communal being as Father and son. That is exactly what is so stunning about adoptive sonship – it is sharing in the Son’s own relationship with the Father: “He who loves me will be loved by my Father.” (John 14:21) There is no adoption, no other way to be children of God, no experience of the fatherly love of God except through the Father’s love for his only begotten Son.’ (150)