Adopted by God for Fellowship with Him

 James Stewart:

"Paul perceived that a Gospel which broke the bondage of legalism, and ended the tyrant sway of principalities and powers, and remitted sin's fearsome penalties, and brought up reinforcements for cowed and beaten human wills, and then stopped there, was no Gospel worthy of the name.  Great and marvelous achievements these all might be; but over and above them all, one thing was needful, one thing without which all the other glories of redemption must remain sterile and unavailing--the restoration of the lost fellowship with God.  Man wants more than the remission of his sins, more than an escape from inward accusations...more than a ransom from the wrath to come.  He wants to be right with God.  He wants to be back in the family again.  He wants, in a word, reconciliation.  Any Gospel that offers itself to a sinning, suffering world must stand and be tested here.  This is the real test.  It is, quite literally, the 'crucial' test:  for at the heart of it lies a cross." – "A Man in Christ"

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.