The Greek is huiothesia, and it only occurs five times in the New Testament, all in the Church Epistles (Rom. 8:15, 23; 9:4; Gal. 4:5; Eph. 1:5). According to Vine it means,
“the place and condition of a son given to one to whom it does not naturally belong.”
The Greek Lexicon by Louw and Nida says:
“to formally and legally declare that someone who is not one’s own child is henceforth to be treated and cared for as one’s own child, including complete rights of inheritance.”
Huiothesia literally means, “to place as a son.” “Adoption” clearly indicates that a Christian is a member of God’s family. In the Roman culture, the adopted son or daughter had four major changes: a change of family, a change of name, a change of home, and a change of responsibilities (Charles Welch, Just and the Justifier. The Berean Publishing Trust, London, p. 212, with a fuller explanation on pp. 208-213).
Most importantly, by using the word “adoption,” God emphasizes that salvation is permanent for the Christian, which is why it appears only in the Church Epistles. Some versions translate huiothesia as “sonship,” but we believe that is not as good as “adoption.” While it is true that someone adopted into the family attains sonship (the status of a son), “adoption” is more accurate to the Greek meaning of the word, and it correctly expresses the fact that the adopted child is permanently placed in the family.
Birth seems so much more desirable than adoption that it is fair to ask why God would even use “adoption.” The answer is that the Romans recognized that when a baby was born, “you got what you got,” whether you liked it or not. This would include the sex of the child, birthmarks, etc. Thus, according to Roman law, a naturally born baby could be disowned from the family.
However, people adopting a child knew exactly what they were getting, and no one adopted a child unless that specific child was wanted as a family member, so according to law an adopted child could not be disowned. He or she was permanently added to the family. Many early believers were Roman citizens, and using the word “adoption” was one of God’s ways to let the Church know that He chose the children brought into His family, and they could not be taken from it.
The Roman historian William M. Ramsay writes:
“The Roman-Syrian Law-Book…where a formerly prevalent Greek law had persisted under the Roman Empire—well illustrates this passage of the Epistle. It actually lays down the principle that a man can never put away an adopted son, and that he cannot put away a real son without good ground. It is remarkable that the adopted son should have a stronger position than the son by birth, yet it was so.”