J. I. Packer:
Paul teaches that the gift of justification (i.e., present acceptance by God as the world’s Judge) brings with it the status of sonship by adoption (i.e., permanent intimacy with God as one’s heavenly Father, Gal. 3:26; 4:4-7). In Paul’s world, adoption was ordinarily of young adult males of good character to become heirs and maintain the family name of the childless rich. Paul, however, proclaims God’s gracious adoption of persons of bad character to become “heirs of God and co-heirs with Christ” (Rom. 8:17).
Justification is the basic blessing, on which adoption is founded; adoption is the crowning blessing, to which justification clears the way. Adopted status belongs to all who receive Christ (John 1:12). The adopted status of believers means that in and through Christ God loves them as he loves his only-begotten Son and will share with them all the glory that is Christ’s now (Rom. 8:17, 38-39). Here and now, believers are under God’s fatherly care and discipline (Matt. 6:26; Heb. 12:5-11) and are directed, especially by Jesus, to live their whole lives in light of the knowledge that God is their Father in heaven. They are to pray to him as such (Matt. 6:5-13), imitate him as such (Matt. 5:44-48; 6:12, 14-15; 18:21-35; Eph. 4:32-5:2), and trust him as such (Matt. 6:25-34), thus expressing the filial instinct that the Holy Spirit has implanted in them (Rom. 8:15-17; Gal. 4:6).
Adoption and regeneration accompany each other as two aspects of the salvation that Christ brings (John 1:12-13), but they are to be distinguished. Adoption is the bestowal of a relationship, while regeneration is the transformation of our moral nature. Yet the link is evident; God wants his children, whom he loves, to bear his character, and takes action accordingly.
—J. I. Packer, Concise Theology: A Guide to Historic Christian Beliefs (Wheaton, IL: Tyndale House, 1993), 167-168.