Father, which Thou to me hast showed?
That I, a child of wrath and hell,
I should be called a child of God.
Charles Wesley wrote those ecstatic words in the grip of wonder at being adopted in Christ as a child of God. J.I. Packer argues that the truth of adoption is the key to unlocking the deepest insights into the gospel and the New Testament's teaching on the Christian life. This is true, he says, even though the word "adoption" appears only five times in the NT. I think Packer is absolutely right. Adoption is the crowning blessing of Christ's saving work, and when Scripture invites us to address God as Father, or think of ourselves as his children, adoption is in the background. It's because of adoption that we can pray, "Our Father who art in Heaven, hallowed by thy name."
One of the few texts that explicitly mentions adoption is Galatians 4:4-7. Previous to this the Apostle Paul has been making the case that faith in Christ, not law-keeping, is what makes Jew and Gentile right with God and true children of Abraham, the exemplar of faith. Flowing out of justification is the blessing of adoption. Through faith we are made "heirs according to promise" and are no longer slaves under the guardianship of the law. The implications are enormous. Here's the key text.
But 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. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" So you are no longer a slave, but a son, and if a son, then an heir through God.
To get the full import of Paul's use of the language of adoption and sonship we need to know a bit about the 1st-century Greco-Roman context. In this society females had no inheritance rights so wealthy men without a son would often adopt a boy from the lower castes to be groomed as an heir. When this boy came of age he would have all the rights that a biological son would have had in that household. Paul announces that in Christ male and female have become one (see Gal. 3:28) and received the full rights of sonship. Further, that we receive the Spirit of adoption who gives us assurance that God our Judge has become God our "Abba" -- the same Aramaic word for father used by Jesus to address his, and now our, Heavenly Father. Justification gives us peace with God, adoption gives us a Father.
In another respect our adoption in Christ is far different from the adoption practiced in Paul's day, or for that matter in our day. It's this aspect that reveals to us the greatness of God's love, and that left Wesley grasping for words. Packer explains in his essential chapter on adoption in Knowing God.
In the ancient world, adoption was a practice ordinarily confined to the childless well-to-do. Its subjects, as we said earlier, were not normally infants, as today, but young adults who had shown themselves fit and able to carry on a family name in a worthy way. In this case, however, God adopts us out of free love, not because our character and record show us worthy to bear his name, but despite the fact that they show the very opposite. We are not fit for a place in God's family; the idea of his loving and exalting us sinners as he loves and has exalted the Lord Jesus sounds ludicrous and wild—yet that, and nothing less than that, is what our adoption means.
Adoption, by its very nature, is an act of free kindness to the person adopted. If you become a father by adopting a son or daughter, you do so because you choose to, not because you are bound to. Similarly, God adopts because he chooses to. He had no duty to do so. He need not have done anything about our sins except punish us as we deserved. But he loves us; so he redeemed us, forgave us, took us as his sons and daughters and gave himself to us as our Father.
Of course God's love doesn't stop there, just as an earthly adoptive parent's love doesn't stop when the legal process is complete. It remains to establish a genuine filial relationship with your son or daughter. You do this by loving the child with the goal of winning the child's love in return. This is exactly what God does. Packer states that the prospect facing the adopted child of God is an eternity of love. Christian, do you see your relationship with God through the lens of adoption? What a difference it makes!
Another implication of adoption is that as children of God we'll want to please our Father by showing forth the family likeness. The Sermon on the Mount gives the fullest picture of what that looks like. Yet even when we mess up God won't cast us out of the family. Only bad fathers do that. He may discipline us as an all-wise father who sees our lives from an eternal perspective, but that's further confirmation of our adoption (see Heb. 12:7, etc).
Discipline, yes. Disinheritance, no. Remember. You are no longer slaves. You are sons. You are heirs.
See what kind of love the Father has given to us, that we should be called children of God; and so we are.
Quote from Knowing God, p. 215