“…adoption is…the highest privilege that the gospel offers: higher even than justification… 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 not 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…And as justification is the primary blessing, so it is the fundamental blessing, in the sense that everything else in our salvation assumes it, and rests on it- adoption included.
But this is not to say that justification is the highest blessing of the gospel. Adoption is higher, because of the richer relationship with God that it involves… Justification is a forensic idea, conceived in terms of law, and viewing God as judge…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, and 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.
J.I. Packer, Knowing God, pp. 186-188