I want to point you to an excellent sermon on Galatians 4:1-7 by C.J. Mahaney, titled God as Father: Understanding the Doctrine of Adoption. Here is one particularly helpful excerpt on the connection and distinctions between justification and adoption:

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… Notice God’s purpose was both to redeem and to adopt — “to redeem those who were under the law, so that we might receive adoption as sons” (v. 5).

I’m sure you will agree that redeeming us from slavery to sin and the penalty of sin would have been sufficiently astounding. But God’s purpose did not conclude with redemption, it culminated with adoption. He made slaves into sons through the death of His Son. And here in this phrase, and this passage, we encounter the deepest insights into the greatness of God’s love!

Now, historically in Covenant Life Church and Sovereign Grace Ministries, we have taught more on the doctrine of justification than we have on adoption. I don’t think we should ever teach less on the doctrine of justification. I do think we should teach more on the doctrine of adoption. Actually, the doctrine of justification must always remain primary because all saving benefits depend on justification by faith alone, through grace alone, in Christ alone. One can’t understand adoption apart from justification. Adoption depends on justification. Grasping justification positions us to fully appreciate adoption.

There are those who speak about the Fatherhood of God without reference to the Cross or the doctrine of justification. We cannot, we should not, and we must not, speak of the Fatherhood of God apart from the Cross and apart from the doctrine of justification.

So with those qualifying remarks let us distinguish between justification and adoption without separating justification and adoption. Let’s distinguish between them because they are not the same thing.

Understanding the differences is of critical importance to experiencing adoption. Dr. J.I. Packer helps us understand the difference and has written the following helpful remarks:

“That justification – by which we mean God’s forgiveness of the past, together with his acceptance for the future – is the primary and fundamental blessing of the gospel is not in question. Justification is the primary blessing, because it meets our primary spiritual need. We all stand by nature under God’s judgment; his law condemns us; guilt gnaws at us, making us restless, miserable, and in our lucid moments afraid; we have no peace in ourselves because we have no peace with our Maker. So we need the forgiveness of our sins, and assurance of a restored relationship with God, more than we need anything else in the world; and this the gospel offers us before it offers us anything else. … But contrast this, now, with adoption. Adoption is a family idea, conceived in terms of love, and viewing God as father. In adoption, God takes us into his family and fellowship – he establishes us as his children and heirs. Closeness, affection and generosity are at the heart of the relationship. To be right with God the Judge is a great thing, but to be loved and cared for by God the Father is a greater” [Knowing God, pp. 206, 207].

I love that last sentence – “To be right with God the Judge is a great thing.” I just want to say it is indeed “a great thing” to be right with God the Judge through the person and work of Jesus Christ. It is “a great thing” to be forgiven of sin. It is “a great thing” to be freed from fear of future wrath. It is “a great thing” to know this day that there is no condemnation for those who are in Christ Jesus. It is “a great thing” to know that on the final day there is no condemnation to those who are in Christ Jesus. To be right with God the Judge – that is “a great thing”!

But to be loved and cared for by God the Father is greater. Now they are inseparable. There is no greater apart from the great. The great precedes the greater. But it’s possible to understand the great and not comprehend and live in the good of the greater.

And if you are right with God the Judge — through the person and work of Jesus Christ — let me just say that is a “great thing”! But as incomprehensible as it is, there is something greater. The greater is to be loved and cared for by God the Father. That’s the greater. This is part of Paul’s burden in this passage, that we not only experience the great (“redeemed”) but the greater (“adoption”).

Do the words closeness, affection, and generosity describe your perception and experience of God? Do they? If not, perhaps you are more aware of your sin than you are the adopting grace of God.

In order to experience more of the love of God, the affection of God, the closeness of God, the generosity of God, I want to recommend that for a season you study the doctrine of adoption until you are assured and secure in the love of God. If you are unfamiliar with the gift of adoption, I want to encourage you to restrict your spiritual diet (if necessary and for a season) to this topic so that you might experience the greatness of God’s love. If you are a Christian and you are not convinced of God’s love for you then I would recommend you confine yourself to this topic. Confine yourself to your study to this passage and other passages that reference adoption. Confine yourself for a season of time to the study of the doctrine of adoption. Immerse yourself in extended study.

— C.J. Mahaney, sermon, “God as Father: Understanding the Doctrine of Adoption” (Dec. 2, 2007) 34:08-41:35.

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Shortly thereafter, C.J. recommended the following books for extended study:

• Knowing God by J.I. Packer (especially ch. 19)
• Children of the Living God by Sinclair Ferguson
• Adopted into God’s Family by Trevor Burke

I encourage you to listen to the full sermon audio here:

Or download the sermon MP3 here.