Answer: Adoption is an act of the free grace of God₁, in and for his only Son Jesus Christ₂, whereby all those that are justified are received into the number of his children₃, have his name put upon them₄, the Spirit of his Son given to them₅, are under his fatherly care and dispensations₆, 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.₇

1. 1 John 3:1
2. Ephesians 1:5; Galatians 4:4-5
3. John 1:12;
4. 2 Corinthians 6:18; Revelation 3:12
5. Galatians 4:6
6. Psalm 103:13; Proverbs 14:26; Matthew 6:32
7. Hebrews 6:12; Romans 8:17

For further study see:

1. Q34: What is adoption?
2. Q36: What are the benefits which in this life do accompany or flow from justification, adoption, and sanctification?
3. WLC 39: Why was it requisite that the Mediator should be man?
4. WCF CHAPTER 12: Of Adoption