(Adoption) is the highest privilege that the gospel offers: higher even than justification. This may cause rising of eyebrows, for justification is the gift of God on which since Luther evangelicals have laid the greatest stress, and we are accustomed to say, almost without thinking, that free justification is God’s supreme blessing to us sinners. Nonetheless, careful thought will show the truth of the statement we have just made….
The free gift of acquittal and peace (justification), won for us at the cost of Calvary, is wonderful enough, in all conscience – but justification does not of itself imply any intimate or deep relationship with God the judge. In idea, at any rate, you could have the reality of justification without any close fellowship with God resulting.
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, p. 206-207