From Ephesians by Peter S. Williamson, commenting on Ephesians 1:5:
The next sentence reveals God’s motive for choosing us—a fatherly love that moved him to adopt us as his children: In love he destined us for adoption to himself. In saying that God “destined us for adoption to himself,” the focus is not on the legal procedure of adoption but on the result of adoption, that is, that we become members of God’s family, his sons and daughters. God intended to accomplish this through Jesus Christ, the only way humans can truly become God’s children. Although Genesis portrays humans as God’s offspring because we were created in his image and likeness (Gen 1–2; see Luke 3:38), sin severely damaged this relationship (see 2:3). When the New Testament speaks of becoming children of God it refers to a far deeper filial relationship with God in Christ and through the gift of the Spirit than was previously possible (John 1:12–13; Rom 8:14–17; Gal 3:26).
Sometimes people are a bit put off by the idea that we are adopted children since this term could seem to distance us from God. But as parents of adopted children can testify, adopted children are not loved less. In the Greco-Roman world of the first century, adopted children enjoyed all the rights and privileges of those born into the family. . . . As children we can count on God’s protection, provision, and steadfast love (Matt 6:31–34; 10:29–31; Rom 8:39). As children, we are “heirs of God,” with a dignity so extraordinary that creation itself will be transformed when our identity as sons and daughters of God is fully revealed (Rom 8:19–21).