What is adoption?
Adoption is an act of the free grace of God, (1 John 3:1) in and for his only Son Jesus Christ, (Eph. 1:5, Gal. 4:4–5) whereby all those that are justified are received into the number of his children, (John 1:12) have his name put upon them, (2 Cor. 6:18, Rev. 3:12) the Spirit of his Son given to them, (Gal. 4:6) are under his fatherly care and dispensations, (Ps. 103:13, Prov. 14:26, Matt. 6:32) admitted to all the liberties and privileges of the sons of God, made heirs of all the promises, and fellow-heirs with Christ in glory. (Heb. 6:12, Rom. 8:17)
The Westminster Larger Catechism: With Scripture Proofs. (Oak Harbor, WA: Logos Research Systems, Inc., 1996).