When God engrafts you into His family, through your repentance and faith, you become a child of God with all the benefits that such a relationship bestows. If I could define this doctrine in a few words, I would say:
The doctrine of adoption in Scripture reveals the Holy Spirit's work of uniting grace-saved sinners with God's family through faith in Jesus Christ, bestowing all the privileges of God's children.
The Westminister Shorter Catechism, question 34, does a much better job:
What is Adoption?
Adoption is an act of God’s free grace, whereby we are received into the number, and have a right to all the privileges of the sons of God.
The doctrine of adoption is presented in these verses from the Apostle Paul:
Romans 8:15 - "For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father."
Ephesians 1:4-6 - "Even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love, he predestined us for adoption to himself as sons through Jesus Christ, according to the purpose of his will, to the praise of his glorious grace."
Galatians 4:4-6 - "But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, 'Abba! Father!'"
The doctrine of adoption in Scripture reveals the Holy Spirit's work of uniting grace-saved sinners with God's family through faith in Jesus Christ, bestowing all the privileges of God's children. — Michael A. Milton
A. A. Hodge of old Princeton described the benefits of divine adoption:1
“Sonship includes (a) derivation of nature (2 Pet. 1:4; John 1:13); (b) the bearing of the divine image or likeness (Col. 3:10; Rom. 8:29; 2 Cor. 3:18); (c) the bearing the Father’s name (1 John 3:1; Rev. 2:17; 3:12); (d) the being the objects of his peculiar love (John 17:23; Rom. 5:5–8); (e) the indwelling of the “Spirit of his Son,” the “Spirit of Adoption” (Rom. 8:15–21; Gal. 4:6; 5:1; 1 Pet. 1:14; Heb. 2:15; 10:19, 22); (f) present protection, consolation, and provision (Luke 12:27–32; John 14:18; 1 Cor. 3:21–23; 2 Cor. 1:4); (g) fatherly chastisement for our good (Ps. 51:11, 12; Heb. 12:5–11); (h) heirship in relation to God and joint heirship with Christ (Rom. 8:17; James 2:5; 1 Pet. 1:4; 5:4).”2
Listen to What is the Doctrine of Adoption, Part II.
Part 1 of the two-part series of the doctrine of adoption is here.
We want to recognize Dr. Jeffrey Waddington's good work at https://www.placefortruth.org/, from which we gathered the quote by A. A. Hodge. Thank you.
Hodge, A. A., J. Aspinwall Hodge. (1888). The System of Theology Contained in the Westminster Shorter Catechism: Opened and Explained. (pp. 66–67). New York: A. C. Armstrong and Son.