Sometimes the Bible refers to adoption on a human level, but the image of adoption as our relationship to the Father is far more important. Three main pictures of adoption are: (1) Israel becoming God's adopted son in the Old Testament. (2) Everyone who believes in Jesus Christ become family as children and heirs. The Spirit leads them to become children of God and confirms that reality in them. (3) Believers looking forward to their full adoption at the resurrection of the dead.
Adoption (huiothesia, "placing as a son"), was a Roman, not a Jewish, practice; not all offspring were heirs. You do not get into God's family by adoption--you get into His family by regeneration: the new birth (Jn 3:1-18; 1 Pet 1:22-25).
Adoption is the act of God by which He gives His "born ones" an adult standing in the family, so that we can immediately begin to claim our inheritance and enjoy our spiritual wealth! An infant cannot legally use this inheritance (Gal 4:1-7), but an adult son can--and should. The future aspect of adoption is in Romans 8:22-23.
In regeneration a Christian receives the nature of a child of God; in adoption he receives the position of a son of God (Jn 1:11). The moment he believes (Gal 3:25-26; 4:6; 1 Jn 3:1,2), the full manifestation of this sonship awaits the resurrection, change, and translation of saints, called the redemption of the body (Rom 8:23; Eph 1:14; 1 Thess 4:14-17; 1 Jn 3:2). There is no procedure to become "unadopted."