Adolf Harnack wrote What Is Christianity to articulate and resolve the issue once and for all of the basis of the faith and came up with the universal fatherhood of God and the universal brotherhood of mankind as the reduction of the doctrines in its LCD or lowest common denomination as the affirmation of a creed. This was too simplistic a breakdown of Christian theology. This view was respected by theologians until the neo-orthodoxy of Swiss theologian Karl Barth writing Church Dogmatics, a tome in itself of four volumes and over 6 million words and considered one of the most pivotal theological achievements of the twentieth century.
"Behold what manner of love the Father has bestowed on us that we should be called the children of God!" (cf. 1 John 3:1). But the Bible says that the unredeemed are the children of the devil, not of God. We must be born again into God's family. Reconciliation means restoration of the broken relationship between us and God as family. God treated us as family and we ought to treat our brethren as members of our true family of God.
The thing about adoption that makes it so compelling to believe in is that it's permanent and gives equal status as that of the natural-born and equal rights as joint heirs. Also, as a metaphor, have you ever heard of anyone becoming "unadopted?" Adoption is a badge of honor and it costs to get adopted; God willing paid this price knowing who we are and what manner of person we'd become! Know that when we are with our church family that we are to be treated as family and can expect this in return from our brethren in Christ. God doesn't treat us as slaves, but as children to whom He has given His will and a Great Commission as ambassadors of Christ. Therefore, we have the spirit in our souls that cries out, "Abba, Father," (cf. Gal 4:6; Rom. 8:15). The word Abba means literally father but Abbi means daddy!
We are already officially adopted and our fate and destiny are sealed by the seal of the Holy Spirit, but it becomes finalized, completed, and actualized in glory. What glory awaits our final adoption with all our inheritance as children of God! We used to be the slaves of sin but now the slaves of righteousness; actually the more enslaved to righteousness we are the freer we become in Christ. Because we were washed and sanctified and we once sold under sin and its power.
We must never ask if we measure up or compare ourselves with others as to whether we are better or worse because God is no respecter of persons and didn't choose us because of our merit or works. We did nothing to deserve it and nothing we can do can take it away; it's a done deal! This grace of adoption and salvation is free but not cheap! It costs the blood of Christ to redeem us!
The call to adoption is inclusive in that the general call of the gospel message goes out to all who will listen; however, many are called but few are chosen! Romans 8 tells us that there is an inner calling of God and whoever God calls gets justified and cannot be lost in the shuffle of the chain of redemption in Romans 8:29-30. Soli Deo Gloria!