The Westminster Shorter Catechism ask and answers the following.
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.
Adoption in our context has some similarities but also some differences from the ancient one. Without the space to expand on the idea we can summarize the similarities as having a new Identity, Community, and Inheritance.
The identity received is that we are made into the image of Christ (Rom 8:29-30). To be adopted by God is to receive a glory lost in the garden. We receive the glory of the Son of God. We put on Christ (our glorified covenant head) and therefore his righteousness is ours. The great exchange spoken of in Scripture where our rags of filth are now robes of royalty (foretold in the garments provided for Adam & Eve). We were slaves before but now we are free. More than freedom we have a Father to whom we cry “Abba! Father!” by the Holy Spirit (Rom 8:15).
Peter says that before we were the Church we were without a community (1 Pet 2:10). In our adoption we find ourselves in the Church, among the redeemed. We are now a community of saints in royal garments at the royal wedding. Each clothed in the glory of the King. This common bond of Christ is what creates the community of the Church.
Today it’s rare that a first-born son is recognized in higher esteem than his other siblings. However, in the culture where Scripture is written, the first-born would be the heir of the estate. In adoption we also receive a new inheritance. We were the outcast but now are sons of God and have that coveted status of co-heirs that comes with the benefits of a new inheritance. In Christ we are “the firstborn son” “heirs according to the promise” (Gal 3:28-29; Rom 8:29).
The beauty of adoption is that God has not just forgiven our sins and declared us righteous. But that adoption, emphasizes the relational aspect of our justification. We are now welcomed into the presence of the King. Before we were not part of the royal court but now we are children of the King and have every right a child of the King has. We are reminded in adoption that we have access and therefore should come to God with everything.
Maybe part of the struggle of faith is believing that we belong to the family of God, and that the King really wants to hear from us. Adoption reminds us that because of Christ, God always calls us to come to Him.