“Jesus Christ is an inexhaustible fountain of blessing to us (Eph. 1:3; 1 Cor. 1:30). When He gives Himself to us that we might enjoy Him, He is not only our justification—the One through whom we experience the forgiveness of sins and the fellowship of His righteousness—He is also our sanctification— the One through whom we are made holy and are transformed into His image.
Yet He is the source of yet another blessing, one so amazing that it would be blasphemous to suggest if it were not true. In our union with Christ, the only begotten Son of God, we participate in what is most precious to Him: His relationship with His Father.
We are, in union with Christ, adopted into the family of God; we become the children, the sons and daughters, of the Most High God. The blessing of adoptive sonship answers another desperate need we have as sinners.
Whereas justification (a forensic benefit) addresses the guilt and condemnation that accompanies sin, and sanctification (a transformative benefit) addresses the depravity and pollution of our nature, adoptive sonship (a familial benefit) addresses our estrangement and alienation from God…
The neglect of adoption in the soteriological understanding of the church is sorely lamentable, for our participation in the sonship of Jesus Christ is indeed basic to the New Testament gospel.
From the biblical teaching on adoption, we learn that we are restored to a familial intimacy with God the Father, through which we are assured of His eternal fatherly care and provision, a love and indulgence that exceeds our imaginations.
We learn that our relationship to God is so radically changed that we go from being ‘children of wrath’ (Eph. 2:3) to His beloved sons and daughters, a relationship in which the Father vouchsafes to care for our every need.
Perhaps even more amazing, we learn that by sharing in the Son we share in His rights as the Firstborn and only begotten Son of God—we are ‘heirs of God and fellow heirs with Christ’ (Rom. 8:17).
Our minds and hearts should surely stagger under the weight of this reality. It simply exceeds our comprehension.
All of this makes defining adoption concisely a rather difficult task. Nevertheless, here is my effort:
Adoption is that benefit of being united to the Son of God through which we share in His sonship with the Father, become the beloved children of God, and enjoy all the privileges and rights of being included in God’s family.”
–Marcus Peter Johnson, One With Christ: An Evangelical Theology of Salvation (Wheaton: Crossway, 2013), 145-146, 147.