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.
It appears how necessary it was that Christ the mediator should be both God & man.
For if He had not been man, He would not have been a fitting sacrifice; and if He had not been God, the sacrifice would not have been sufficient.Marrow of Theology
Justification, of course, has no degrees and is completed at one moment and in only one act. Yet, in manifestation, consciousness, and effects it has many degrees.The Marrow of Theology, 161
The first act of religion, therefore, concerns those things which are communicated to us from God. The other concerns those things which we yield to God.
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.