What does the Bible mean by adoption?
In Scripture, adoption means the placement of a son as a full operating son of the Father, on equal basis with Him in responsibility, function, and everything—not the modern legal transfer of a child from another union. Paul uses it as a schoolmaster concept: the Law was meant to prepare Israel to receive the Messiah.
Les explains that biblical adoption is not modern adoption, where a child is legally taken from another union and brought into a family. Instead, it was a rite of passage: when a father agreed his son was ready to join him in business, the son was placed as a full operating son, equal with the Father in responsibility, function, and everything. Paul uses this term as a schoolmaster—a hired servant instructed to bring the child to the place where he could step in and work with the Father. That is what the Law was intended to do for Israel: to prepare them so that when Messiah came, they could believe and say yes. A few did, but the nation mostly rejected it. Now this concept comes into the Gentile economy, where the Law was given to prepare everything for Messiah's coming, that He could fulfill the work of the Cross—He had to die, be buried, and be raised for our justification.