In Knowing God J. I. Packer writes:

If you want to judge how well a person understands Christianity, find out how much he makes of the thought of being God’s child, and having God as his Father. If this is not the thought that prompts and controls his worship and prayers and his whole outlook on life, it means that he does not understand Christianity very well at all. (p. 201)

In his Concise Theology he writes:

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 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. (pp. 167-168)

At the recent 2015 Ligonier National Conference, there was a special moment when R. C. Sproul Jr. brought his son onstage and talked about the beauty of adoption. You can watch it below:

For more on adoption, see the forthcoming revised and expanded edition of Russell Moore’s classic, Adopted for Life: The Priority of Christian Adoption for Families and Churches (due out October 2015) and the booklet drawn from one of the chapters, Adoption: What Joseph of Nazareth Can Teach Us about This Countercultural Choice (due out in June).