Shorter Catechism Q. 34. What is adoption? A. 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.
Commentary: One of the benefits of our union and communion with Christ that we receive in this life is adoption. This benefit implies that prior to our union with Christ we were not God’s children. Rather, we were by nature children of the devil (John 8:44), children of wrath (Eph. 2:3), enemies of God in our minds (Col. 1:21), rebellious (Eph. 2:2), dead in our trespasses and sins (Eph. 2:1), and God-haters (Rom. 1:30). Nevertheless, 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. (Gal 4:4-5).
Being a child of God means that we have the right to certain benefits and blessings. The Westminster Larger Catechism 74 enumerates some of them: “[They] have his name put upon them, the Spirit of his Son given to them, are under his fatherly care and dispensations, 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.”