Adoption is the more specific idea introduced here and was a very significant concept in the ancient world, employed by Paul with intent. Under Roman law, adoption bestowed all the privileges of naturally born children, with one very famous example: Gaius Octavian Thurinus was adopted by Julius Caesar and became Augustus Caesar—the first Roman emperor. The ancient world could hardly imagine a greater endorsement of adoption than that. But Paul did.Pillar Commentary
If you want to judge how well a person understands Christianity, find out how much he makes of the thought of being God's child, and having God as his Father. If this is not the thought that prompts and controls his worship and prayers and his whole outlook on life, it means that he does not understand Christianity very well at all.
For everything that Christ taught, everything that makes the New Testament new, and better than the Old, everything that is distinctively Christian as opposed to merely Jewish, is summed up in the knowledge of the Fatherhood of God. 'Father' is the Christian name for God. Our understanding of Christianity cannot be better than our grasp of adoption.Knowing God (??)
Adoption is the highest privilege of the gospel. The traitor is forgiven, brought in for supper, and given the family name. To be right with God the Judge is a great thing, but to be loved and cared for by God the Father is greater.
o how great a task is it, for a poor soul that becomes sensible of sin, and the wrath of God, to say in Faith, but this one word, Father! I tell you, however hypocrites think, yet the Christian, that is so indeed, finds all the difficulty in this very thing, it cannot say, God is its Father.
Adoption is the gracious act of God wherein He makes justified sinners His beloved children. The Westminster Confession says to be adopted is to receive God's name and to have access to God's throne, His pity, His protection, His provision, His discipline, and His promise to never abandon us.