Whenever “adoption” appears in Scripture, my ears pop open. This past holy day’s second reading from Galatians:
But when the fullness of time had come, God sent his Son, born of a woman, born under the law, to ransom those under the law, so that we might receive adoption.
The Greek hyiothesia refers to legal adoption in the Greco-Roman world. Adoption was a final legality in which a person was fully absorbed into the household, never to be reversed into slavery, being disowned, losing rights as an heir, and the like. That would be our Christian understanding of our adoption (as Gentiles) into the family of God.
Paul’s teaching is that the Holy Spirit raises us up from other considerations; the Spirit inspires our uttering of “Abba,” or “Father.” This point is developed a bit more in Romans 8 and 9.